Abstract:
An optical combiner ( 25 ), comprising a bundle of input fibers ( 24 ) spliced to an output fiber ( 26 ), the output fiber having a cladding and at least one high-index portion within the cladding, such that the high index portion has a diameter substantially equal to or less than the outer diameter of the input fiber bundle at the splice point.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 application of International PCT Application No. PCT/GB2014/050203 filed on Jan. 27, 2014, which claims priority to Great Britain Application No. GB 1301745.4, filed on Jan. 31, 2013. The contents of both of these priority applications are hereby incorporated by reference in their entireties. 
     TECHNICAL FIELD 
     This invention relates to a fibre optical laser combiner. In particular, it relates to a combiner for combining the output from several lasers into a single output fibre, and apparatus and methods for controlling the spatial beam profile emitted from that fibre. 
     BACKGROUND 
     Many laser-processing schemes rely on beam delivery via an optical fibre. This fibre is usually radially symmetric (of circular cross-section) and has a uniform refractive index profile (otherwise known as step-index). The beam emitted from such a fibre is thus also circularly symmetric and produces a generally uniform distribution of light on a workpiece receiving a laser beam via the beam delivery optical fibre. 
     For many applications it is desirable to produce tailored non-uniform light distributions on the workpiece, such as an annular profile or profiles having a central peak. Schemes are available for producing such profiles but are often complex and involve the use of free-space optics. The use of these is undesirable, particularly with high-power fibre-laser systems. 
     One method of producing high power fibre laser systems is to combine the outputs from several lasers via a tapered fibre bundle, spliced to an output fibre. Each laser is delivered to the bundle via a separate input fibre, the laser beams in the separate input fibres are then combined and all their inputs exit via the same single output fibre. These are known generally as output combiners. One aspect of such a combining scheme is that, although the individual input fibres are located in close proximity to each other, the inputs are distinct while they remain in the tapered bundle. The standard output fibre collects all of the inputs and produces a uniform output as all the inputs are overlapped by the same circularly symmetric single refractive index region. 
     WO 2011/048398 discloses a system having a tapered input fibre bundle. 
     SUMMARY 
     The present invention arose in an attempt to provide a combining arrangement which can produce a non-uniform, or tailored light distribution at a workpiece. 
     According to the present invention in a first aspect there is provided an optical combiner, comprising a bundle of input fibres spliced to an output fibre, said output fibre comprising a first region with refractive index n0 and diameter equal to or greater than the input fibre bundle diameter and one or more secondary regions within the first region, the second regions each having refractive index that differs from n0, each of the secondary regions not overlying all of the input fibres. 
     The secondary regions are thereby arranged such that they provide coupling from only a subset of the input fibres. 
     A secondary region is said to overlie an input fibre if the end face of the input fibre is enclosed or partially enclosed within the end face of the secondary region. 
     The first region has a diameter which is preferably equal or substantially equal to the diameter of the input fibre bundle at the splice point. 
     The output fibre may be a double-clad output fibre. 
     The first region may be a cladding. 
     The secondary region may comprise a central core. 
     The secondary region may alternatively or in addition comprise one or more annular regions. 
     Where the secondary region comprises at least one annular region, the input fibre bundle comprises at least one radially outer set of input fibres, and said annular region overlies said radially outer input fibres. 
     The output fibre may have a secondary region surrounded by a first region, and the input fibre bundle may have a central fibre and a plurality of fibres radially surrounding this, such that the secondary region of the output fibre is of diameter equal to or less than the diameter of the central input fibre. It is preferably positioned with its entry face lying within the area defined by the output face of said central input fibre. It may be co-axial therewith. The secondary region in this or other embodiments may be circular or elliptical, for example, or have other shapes. 
     Other configurations of the output fibre may be used. In embodiments in which an output fibre has a central secondary region and surrounding first region, e.g., a central core and a surrounding cladding, it is observed that the central core acts to capture the majority of the light from a central input fibre and thus tends to give a pronounced peak in the centre of an output beam profile. Furthermore, a portion of light from the outer input ‘port’ (i.e., input fibres) can also be captured by the central core of the output fibre. This can lead to a profile having a central peak, which is beneficial for a range of laser processing operations. 
     In embodiments in which an output fibre has an annular secondary region, such as a circular high-index ring that overlaps with the outer fibres of the input bundle, then a majority of the input light is coupled directly to the annular pedestal which is formed by the high-index ring. This leads to an intensity profile of light output against the diameter which has an annular peak. This method of producing an annular beam is a robust and simple method compared to other bulk optic schemes. Furthermore, the brightness increases, typically by the ratio of the overall fibre area to the annular pedestal area. Such a profile is also beneficial for a wide range of laser processing applications. 
     In a further aspect, the invention provides a method of providing a single output from a plurality of lasers, comprising providing an input fibre bundle having a plurality of input fibres receiving laser outputs from each of a plurality of lasers, and splicing the bundle, at a splice point, to a single output fibre; said output fibre comprising a first region with refractive index no and diameter equal to or greater than the input fibre bundle diameter and also includes one or more secondary regions within the first region, the secondary regions each having refractive index that differs from no, each of the secondary regions not overlying all of the input fibres. 
     The secondary regions may be an annular region or a plurality of annular regions, a combination of a central core and one or more annular regions, or other configurations where more than one high-index region is provided, which regions may be of different refractive indices. 
     In a further aspect, the invention provides a laser system including an output combiner as described. 
     In a further aspect, the invention provides a method of material processing, or of tailoring a beam profile during material processing, using a method or apparatus as described. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: 
         FIG. 1  shows an end view of input fibre bundle; 
         FIG. 2  shows a matching output fibre without a secondary region; 
         FIG. 3  shows a plot of output intensity; 
         FIG. 4  shows an end view of a fibre bundle; 
         FIG. 5  a matching output fibre with a secondary region; 
         FIG. 6  shows a plot of output intensity with edge input; 
         FIG. 7  shows a plot of output intensity with a centre input; 
         FIG. 8  shows an end view of an input bundle; 
         FIG. 9  shows a matching output fibre a secondary region; 
         FIG. 10  shows a plot of output intensity; 
         FIG. 11  shows an output fibre similar to  FIG. 5 ; 
         FIG. 12  shows the output fibre overlapped with a tapered input fibre bundle; 
         FIG. 13  shows an annular high-index region output fibre similar to that of  FIG. 9 ; 
         FIG. 14  shows the output fibre of  FIG. 13  overlapped with a tapered input fibre bundle; and 
         FIG. 15  shows a system usable for material processing. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 to 3  show a previously proposed system. An input fibre bundle comprises a bundle of seven fibres comprising a first central fibre  1  and six outer fibres  2   a  to  2   f . Each fibre has a cladding  4  (of diameter d) and a core  3 . The fibre bundle is tapered in a known manner. It receives inputs from seven separate fibre lasers at a proximal end and the distal end is shown in the figure, from which the individual laser outputs are emitted via the separate fibres. This is spliced to a matching output fibre  5  typically of cladding diameter 3d. In the output fibre, the individual outputs from the separate laser, which have been applied through the individual fibres of the fibre bundle shown in  FIG. 1  are combined the resulting beam is output at the output end of the output fibre  4 .  FIG. 3  shows approximately the relative intensity of the output across the diameter of the output phase of the fibre and it will be seen that this is generally uniform across the entire diameter. Of course, the diagram is simplified and there may be slight variations in practice. 
       FIG. 4  again shows a similar input fibre bundle to that of  FIG. 1 . Note that the input fibre bundles themselves are well known and comprise a central core  3  and an outer cladding region. This is spliced to an output fibre  6  shown in  FIG. 5  which differs from that of  FIG. 2  by having a central core  7  and a surrounding cladding  8 . Thus, the cladding region  8  is a first region of refractive index n0, and the core  7  is a secondary region of index n1, different to n0. The cladding diameter is approximately equal to the outer diameter of the tapered input fibre bundle and thus is of diameter approximately 3d. The central core is of higher refractive index than the cladding. In one embodiment, the refractive indices are as follows: 
     Core=1.459 
     Cladding=1.455 
     NA=0.11 
       FIG. 11  also shows the output fibre of  FIG. 5  and  FIG. 12  shows this superimposed upon a cross-sectional view of the tapered input fibre, illustrating the individual input fibres  11   a  to  11   g  and the output fibre  12  having a core  13  which lies generally concentric (coaxial) with, or at least inside (or coextensive with) a central fibre  11   g . It is observed that the inner core ( 7 ,  13 ) acts to capture the majority of light from the central input fibre  11   g  and thus gives a pronounced peak  15  in the output beam profile which is shown in  FIG. 6 . Furthermore, it is also observed that a portion of the light from the outer input fibre (input ports)  11   a  to  11   f  is also captured by the central core of the output fibre. Thus, a profile as shown in  FIG. 6  or in  FIG. 7  for example with a central peak  15  above a plateau level  16  is obtained. Such a profile is beneficial for a range of laser-processing operations. 
       FIG. 7  shows an example of a centre input (ie where the input comes mainly or wholly from the central input fibre) and  FIG. 6  shows an example of an edge input, in which the majority of the input comes from the ring of fibres surrounding the central input fibre. It is seen that with a centre input a much more pronounced peak is obtained but a significant peak is still obtained with an edge input. By varying the type of input and also the index and size of the central core and/or cladding different outputs can be obtained for different uses. 
       FIG. 8  again shows an input fibre bundle similar to that of  FIG. 4 . 
       FIG. 9  shows an output fibre  17  having an annular relatively high-index region  18 .  FIG. 13  shows an annular high-index region output fibre similar to that of  FIG. 9 , and  FIG. 14  shows the output fibre of  FIG. 13  overlapped with a tapered input fibre bundle. As is shown in  FIG. 14 , this most preferably overlaps the outer fibres ( 11   a  to  11   f ) of the input tapered fibre bundle shown in  FIG. 8 . That is, the inner diameter d1 is greater than or equal to d (the diameter of an input fibre) and the outer diameter d2 of the annular region is less than or equal to 3×d, as is shown in  FIG. 14  where the annulus is shown as being overlapped with the outer input fibres. 
     This leads to an output profile as shown schematically in  FIG. 10  having an annulus  20 ,  21  above a plateau  22 . In such an output fibre which has an annular high-index region that overlaps with the outer fibres of the tapered input bundle, the large majority of the input light is coupled directly to this annular pedestal. The efficiency of the system tends to be very high compared with free space methods of annular beam generation. In addition, it is found that the brightness of the source actually increases (by the ratio of the overall fibre area to the annular pedestal area). 
       FIG. 15  shows a typical application of the invention in a material processing application. The laser sub units  23  shown may have an output power of up to 1.5 kW and the combined beam at  26  may be up to 10 kW. The laser source from combiner  25  is directed via coupling optics  27  onto the material to be modified  28 . 
     N individual fibre laser sources  23  are coupled into the combiner  25  through their respective feed fibres  24 . The combiner is formed by the fusion of the feed fibre  24  and the delivery fibre  26 . Through the choice of refractive index profile of the delivery fibre  26  and the orientation of the feed fibres  24  relative to this fibre the output beam profile at  28  can be controlled as described earlier in this application. Examples of the profiles are shown in  FIGS. 6, 7 and 10 . 
     A further aspect of this invention is fast switching of the mode profile. By individually addressing/controlling the component lasers  23  the output beam profile at  28  can be switched. For example using the combiner described in  FIGS. 4 and 5  excitation of all the lasers produces a broad near flat top profile ideal for welding and thick section cutting. Excitation of just the central port laser on the other hand produces a narrow beam profile which is ideal for thin section cutting. Thus, each laser may be switched ON or OFF, during a material processing operation, independently of the other laser, to alter or tailor the beam profile. The time to switch between these two profiles is limited by the response time of the control electronics for the individual lasers  23 . Typically this can be of the order of tens of microseconds. This time is far faster than alternative bulk optic switching methods that have been used previously to control the beam profile. This rapid switching time enables the possibility of in process beam profile switching for optimised material processing. 
     One, two or more of the N lasers may be turned ON or OFF, or their output varied, to alter the beam profile. 
     The embodiments shown and described are illustrative only and other embodiments may be used. Some may have a central core and one or more annular or other shape regions of relatively high-index compared to the rest of the output fibre. Other shapes may be used for different beam profiles.