Abstract:
A method for performing a multi-stage dilation of optical fibres is described, the method comprising performing successive dilation steps such that the adiabatic condition is maintained throughout the fibre. There is also described various optical devices employing such multi-stage dilated optical fibres, as well as methods of manufacture of the optical devices.

Description:
[0001]    This application is a 35 U.S.C. 371 national phase filing of PCT/EP2010/063151, filed Sep. 8, 2010, which claims priority to Irish national application number S2009/0787 filed Oct. 9, 2009, the disclosures of which are incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to optical devices and methods of manufacture thereof, in particular single-mode optical fibre devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical fibres that guide light by total internal reflection consist of a cylindrical core that has a higher refractive index than the surrounding cladding. For single-mode operation, the core size and the index difference between the core and cladding are such that only the fundamental mode is propagated for a given spectral bandwidth, as determined by the characteristic waveguide number or V-number. (A single mode fibre is an optical fibre that is designed for the transmission of a single ray or mode of light.) 
         [0004]    With reference to  FIG. 1 , an example of a single-mode optical fibre system is indicated at  10 . The system  10  comprises a single-mode optical fibre  12  having an internal core  16  within external cladding  18 . The core  16  and cladding  18  are protected within an external buffer  14 , which is shown as being stripped along the length of the fibre that is to be processed. A cross-section of the fibre  12  across line A-A is indicated at  20 . Typical dimensions for a standard telecommunications optical fibre  12  would be 9 μm diameter for the core  16  and 125 μm diameter for the cladding  18 . 
         [0005]    In the case of high-power transmission through standard single-mode fibres, end terminations of fibres and in-line splices or interconnects can introduce undesirable back-reflections and facet distortions that can lead to system damage and failure. Further complications can arise due to, e.g. dirt at the end termination and/or between the end of the fibre and an associated connector. In order to reduce this problem, it is desirable to reduce the power density by expanding the mode field diameter. This can be achieved using a variety of techniques, including fibre tapering, thermal core diffusion, lensing including bulk and grin lensing, fibre end shaping, and splicing on dissimilar fibres including e.g. multimode fibres. However, where a typical beam diameter of ˜50 μm is desired, each of these solutions has associated problems. 
         [0006]    In the case of tapering, the fibre becomes small, difficult to handle and more sensitive to external influences—making it difficult to package. The diffusion approach is limited in the extent to which the beam may be expanded before loss becomes significant. Lensing does not reduce the optical power density at the fibre end-face, generally involves the introduction of free-space facets, back-reflections, glues, alignment issues and loss within in-line fibre pigtailed bulk-optic sub-systems, and is expensive. Using dissimilar fibres requires a splice and introduces back-reflections and loss where the beam diameter is not mode-matched, and it can be a relatively expensive process compared with the approach described here. 
         [0007]    An alternative technique which has been recently proposed is that of fibre fattening (also referred to as fibre up-tapering or fibre dilation), discussions of which may be found in [1] PhD thesis, Elaine M. O&#39;Brien, Lightwave Technology Research Centre, University of Limerick; [2] “Up-tapering of optical fibres using a conventional flame tapering rig”, G. Kakarantzas, L. Prill-Sempere and P. St. J. Russell, CFK2, Optical Society of America-CLEO/QELS Conference, 2007; and [3] “Adiabatic dialated standard and speciality optical fibres”, N. Healy, D. F. Murphy, E. M. O&#39;Brien and C. D. Hussey, Poster080 Photonics Ireland 2007 (Galway), which are incorporated herein by reference in their entireties. 
         [0008]    In known fibre fattening processes, a fibre to be fattened is positioned between a pair of holders, and a heat source is applied along a length of the fibre to soften the core and cladding material. The heat source may be a conventional flame, or could comprise an arc, laser, or other heat source. The action of heating a fibre that is subjected to a compressive force above its glass transition temperature results in the expansion of the width of the fibre in conjunction with a reduction of the fibre length. 
         [0009]    An example of the effects of up-tapering is shown in  FIG. 2 , which shows fibre  12  after up-tapering has taken place. As can be seen from  FIG. 2 , the length of the fibre  12  has decreased, the newly-fattened fibre  12   a  now showing transitions  22  between the end portions of the fibre  12   a  and the expanded middle portion  100  of the fibre  12   a . An indication of the cross-section of the fibre  12   a  along line B-B is indicated at  24 . Typical dimensions of the expanded cross-section after up-tapering would be 30 μm diameter for the core  16  and 375 μm diameter for the cladding  18 . 
         [0010]    Such up-tapered fibres provide for numerous advantages, e.g. the reduction of optical power density, the improvement of mode-matching between spliced dissimilar fibres, and the flattening of the wavelength response of fused directional fibre couples. 
         [0011]    The up-tapering process is limited by a number of conditions which must be satisfied:
       1. The adiabatic condition needs to be satisfied—i.e. the transition between the fattened and non-fattened sections needs to be sufficiently smooth to ensure the launch of only the local fundamental mode, so as to avoid any losses due to the transition. For the transitions to be adiabatic, at any point along the processed fibre, the transition must satisfy the slowness criterion:       
 
         [0000]    
       
         
           
             
                
               
                 
                    
                   a 
                 
                 
                    
                   z 
                 
               
                
             
              
             
               a 
               
                 z 
                 b 
               
             
           
         
       
       
         
           
              This is known as the adiabatic condition, wherein a is the core radius at any position z along the transition such that da/dz defines the taper angle and z b  is the beat length or period of power oscillations between the excited modes of the system. The shortest beat length can be considered as that between the HE 11  mode (i.e. the designation for the fundamental mode of an optical fibre) and the closest mode of the same symmetry, the HE 12  mode. Transition losses due to non-compliance with the adiabatic condition are one of the more considerable limitations in fibre up-tapering. 
             2. A waveguide needs to be maintained. In conventional optical fibres, light is guided by total internal reflection, which is made possible by the index difference between the core and cladding. In general, the cladding used is silica, and the core has a raised index that is achieved by doping silica with germanium. The heating of the fibre during the fattening process results in thermal diffusion of the core dopant, germanium. With diffusion, the index difference between the cladding and core is reduced and the waveguide becomes weaker. Unless the diffusion is controlled, the diffusion may occur to such a degree that there will effectively no longer be an index step, and the optical fibre no longer acts as a waveguide. Further, any diffusion that does occur needs to satisfy the adiabatic condition given in 1. above. 
             3. Physical size mismatch. As a fibre is fattened, the fattened section becomes larger and heavier to the point that the standard fibre leads are no longer able to support its weight, and an inevitable sagging will take place. 
             4. Mode-area limit. Taking the example of large mode area fibres for lasing, as the mode area is increased, the fibre&#39;s ability to maintain single-mode only propagation is reduced, and light couples into the other modes of the fat, and accordingly highly multimode, structure. A number of techniques can be used to strip out higher mode behaviour and therefore maintain single-mode operation. This is a minimal concern in cases where the fattened fibre section is fattened over a short length. 
           
         
       
     
         [0016]    Accordingly, current fibre fattening techniques are limited to the expansion that can be achieved, typically up to ˜2.25 times dilation of the original fibre. It is an object of the invention to provide a new method of fibre fattening method that allows for greater dilation of fibres, while satisfying the limitations described above. 
       SUMMARY OF THE INVENTION 
       [0017]    Accordingly, there is provided a fibre dilation method for providing a multi-stage dilated optical fibre, the method comprising the steps of:
       performing a first-stage fibre dilation process on an optical fibre to form a first dilated section of the optical fibre; and   performing a second-stage fibre dilation process on said first dilated section of the optical fibre to form a second dilated section of the optical fibre, wherein said second dilated section is arranged such that the transition in diameter formed between said first dilated section and said second dilated section is spaced from the transition in diameter formed between the undilated section of the optical fibre and said first dilated section.       
 
         [0020]    As the transitions between successive stages are spaced from one another, the adiabatic condition can be satisfied, and the transition losses kept within acceptable limits. The spacing is chosen to prevent significant transition losses between stages. 
         [0021]    Preferably, said fibre fattening process comprises:
       applying a heat source along a portion of the length of an optical fibre to soften said portion of the optical fibre; and   applying a compressive force to said portion of the optical fibre to dilate said portion.       
 
         [0024]    Preferably, the method further comprises the steps of iteratively performing at least one successive fibre dilation process on the dilated section of a preceding stage, wherein the transition formed by said at least one successive fibre dilation process is spaced from the transition formed by the preceding stage. 
         [0025]    As the transitions are spaced between successive stages, then significant transition losses between stages are prevented from occurring. 
         [0026]    Preferably, the spacing is chosen such that the adiabatic condition is satisfied. 
         [0027]    The adiabatic condition states that: 
         [0000]    
       
         
           
             
                
               
                 
                    
                   a 
                 
                 
                    
                   z 
                 
               
                
             
              
             
               a 
               
                 z 
                 b 
               
             
           
         
       
     
         [0000]    wherein a is the core radius at any position z along the transition in diameter such that da/dz defines the taper angle and z b  is the beat length between the HE 11  and HE 12  modes. HE 11  is the designation for the fundamental mode of the optical fibre, with HE 12  being the closest mode of the same symmetry. 
         [0028]    Preferably, the spacing between successive transitions is 5 mm. This would be preferable for situations where a stationary oxy-butane flame is used as the heat source. In the case of a laser heat source being used, a smaller spacing may be preferred, due to the sharper thermal edges of the heat source. With a sophisticated tapering rig apparatus and moving a flame/laser heat source, the spacing between successive transitions could be reduced to a quasi-continuous transition or “zero-spacing”, rather than a step transition. 
         [0029]    Preferably, said first-stage fibre dilation process comprises dilating a section of said optical fibre to 2-3 times the diameter of said optical fibre, further preferably, 2.25 times the diameter. 
         [0030]    Preferably, said second-stage fibre dilation process comprises dilating said first dilated section of the optical fibre to approximately 4-5 times the diameter of said original optical fibre, further preferably, 4.5 times the diameter. 
         [0031]    There is further provided a multi-stage dilated optical fibre manufactured according to the above method. 
         [0032]    The invention further provides for a method for the low-loss coupling of standard optical fibres with large mode area optical fibres, the method comprising the steps of:
       manufacturing a multi-stage dilated optical fibre;   cleaving said multi-stage dilated optical fibre to provide a large mode area cleaved end; and   splicing said cleaved end of said multi-stage dilated optical fibre to a large mode area optical fibre.       
 
         [0036]    In general, a standard fibre is multi-stage dilated up to the point of optimum mode area matching with a large mode area fibre, e.g. a high-power fibre laser type fibre. Then the multi-stage dilated standard type fibre is cleaved at the dilated section and spliced to the large mode area fibre to form a low-loss interface between the large mode area fibre and the standard fibre through the dilation of the standard fibre. In addition or alternatively, the large mode area fibre may be tapered down to match the dilated section of the standard fibre. 
         [0037]    Preferably, said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre. 
         [0038]    Preferably, the method comprises the step of selecting a large mode area optical fibre such that the diameter of the core of said large mode area optical fibre substantially corresponds to the diameter of the core of said cleaved end. 
         [0039]    The invention further provides an optical fibre comprising a portion of standard optical fibre and a portion of large mode area optical fibre coupled according to the above method. 
         [0040]    The invention further provides for a method of manufacture of an optical wavelength converter, the method comprising the steps of:
       manufacturing a multi-stage dilated optical fibre;   cleaving said multi-stage dilated optical fibre to provide a first cleaved end of a multi-stage dilated optical fibre;   providing a large mode area doped optical fibre having a first end and a second end;   coupling the core of said cleaved end of a multi-stage dilated optical fibre with the first end of the core of said large mode area doped optical fibre, such that said cleaved end of a multi-stage dilated optical fibre and said large mode area doped optical fibre form an optical wavelength converter operable to convert optical signals of a first wavelength received at the uncleaved, undilated end of said multi-stage dilated optical fibre to optical signals of a second wavelength at said second end of said large mode area doped optical fibre.       
 
         [0045]    Preferably, said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre 
         [0046]    Preferably, the method comprises the step of selecting a large mode area optical fibre such that the mode diameter of the core of said large mode area optical fibre substantially corresponds to the mode diameter of the core of said cleaved end. 
         [0047]    The mode diameter/area is governed both by the physical dimensions of the fibre and by the index step between the core and the cladding. It is possible to have matched physical diameters but unmatched modes. To optimise the match between the modal areas, both the physical diameter and index step size should be considered. 
         [0048]    The invention further provides an optical wavelength converter manufactured according to the above method. 
         [0049]    The invention further provides for a method of manufacture of an optical fibre amplifier, the method comprising the steps of:
       manufacturing a multi-stage dilated optical fibre;   cleaving said multi-stage dilated optical fibre to provide a first and a second section of a multi-stage dilated optical fibre having respective first and second cleaved ends and uncleaved ends;   providing a large mode area doped optical fibre having a first end and a second end;   coupling the core of said first and second cleaved ends of said multi-stage dilated optical fibre sections with the core of the respective first and second ends of said large mode area doped optical fibre to form an optical fibre amplifier, the amplifier operable to amplify an optical signal transmitted between said first and second uncleaved ends through said multi-stage dilated optical fibre sections and through said large mode area doped optical fibre.       
 
         [0054]    Preferably, wherein said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre. 
         [0055]    The invention further provides an optical fibre amplifier manufactured according to the above method. 
         [0056]    The invention further provides for a method of manufacture of an optical fibre laser, the method comprising the steps of:
       manufacturing a multi-stage dilated optical fibre;   cleaving said multi-stage dilated optical fibre to provide a first and a second section of a multi-stage dilated optical fibre having respective first and second cleaved ends and uncleaved ends;   processing said first and second cleaved ends to provide reflecting means at said cleaved ends;   providing a large mode area doped optical fibre having a first end and a second end;   coupling the core of said first and second cleaved ends of said multi-stage dilated optical fibre sections with the core of the respective first and second ends of said large mode area doped optical fibre such that a resonant lasing cavity is provided between said first and second cleaved ends to form an optical fibre laser.       
 
         [0062]    For lasing to occur, a resonant gain cavity is required in the fibre. This cavity is excited by a pump source and resonance is achieved in the cavity, at the lasing wavelength, using reflectors at either end of the cavity. In a fibre system, the end reflectors are typically fibre Bragg gratings—the most convenient and effective and preferred method. However, further types of reflecting means may be employed, for example, it would be possible to “drill” into the fibre either side of the lasing cavity and deposit silver/gold to achieve end reflection. 
         [0063]    Preferably, said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre. 
         [0064]    Preferably, said step of processing comprises providing partial end reflectors in both cleaved ends. Preferably, said step of processing comprises inscribing a grating pattern at said cleaved ends. Preferably, said grating pattern comprises a fibre Bragg grating. 
         [0065]    The invention further provides an optical fibre laser manufactured according to the above method. 
         [0066]    Preferably, the fibre is selected from one of the following types of glass: phosphate, silica, telluride, fluoride, chalcogenide. 
         [0067]    Preferably, the fibre is doped with a rare-earth material. Preferably, the fibre is doped with one of the following dopants: erbium, thulium, chromium, ytterbium, neodymium, praseodymium, terbium, or a combination thereof. 
         [0068]    It will be understood that the general terms fattening, dilation, expansion and up-tapering are interchangeable, and are used to refer to the expansion of the diameter of an optical fibre as described by the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0069]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0070]      FIG. 1  is a view of a known optical fibre system; 
           [0071]      FIG. 2  is a view of the optical fibre of  FIG. 1  after single-stage up-tapering has been performed; 
           [0072]      FIG. 3  is a view of the optical fibre of  FIG. 1  after two-stage up-tapering has been performed according to the invention; 
           [0073]      FIG. 4  shows a series of optical devices that can be manufactured using the up-tapered fibre of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0074]    The invention relates to a multi-stage fibre expansion process. For the first-stage fattening, a portion of optical fibre  12  stripped of its buffer  14  (as shown in  FIG. 1 ) is held taut between a pair of vacuum chucks. A heat source, e.g. an oxy-butane flame burner, having a length shorter than that of the optical fibre  12  is applied along a portion of the length of the fibre  12 , while a compressive force is applied to the fibre  12 . 
         [0075]    In general, the heat source may comprise a relatively long flame, the length of which corresponds to the length of that portion of the fibre it is desired to fatten. Alternatively, the heat source may comprise a relatively short flame which is swept back and forth along the length of that portion of the fibre  12 . 
         [0076]    As the heat source softens the material of the optical fibre, the compression acts to dilate or fatten the body of the fibre  12 . This process is repeated until the limitations regarding fibre fattening or up-tapering, e.g. transition losses, start to become significant. In most optical fibre situations, this would approximate to the point where the middle portion of the original fibre  12  has expanded to roughly 2-3 times the original cross-sectional area, as indicated by the section  100  of the fattened fibre  12   a  of  FIG. 2 . 
         [0077]    A second-stage fattening is now performed, wherein a heat source is applied along a portion of the expanded section  100  of the fattened fibre  12   a . The heat source for the second stage fattening is applied along a shorter length of the fibre  12   a  than the heat source for the first stage, with the result that the second-stage fattening occurs away from the transitions  22  in optical fibre diameter caused as a result of the first-stage fattening. As with the first stage fattening, a compressive force is applied to the fibre  12   a , resulting in the dilation of the heated portion of the fibre  12   a . As with the first fattening stage, the second fattening stage can be performed until the limitations regarding fibre fattening or up-tapering start to become significant, or until the desired dilation is achieved. 
         [0078]    With reference to  FIG. 3 , a second-stage fattened optical fibre is shown at  12   b . As a result of the second fattening stage, the expanded section  100  of the first-stage fattened fibre  12   a  has effectively shortened in length to section  100   a , with a second expanded section  102  formed within the boundaries of section  100   a  having an increased diameter than that of the fattened section  100  of the first-stage fattened fibre  12   a . As can be seen from  FIG. 3 , the transitions in diameter  26  between the first-stage fattened fibre  100   a  and the second-stage fattened fibre  102  are spaced from the transitions in diameter  22  between the original unfattened optical fibre  12  and the first-stage fattened fibre  100   a . As the fattened sections  100   a , 102  are arranged such that a space is maintained between the transitions  22 , 26 , this prevents the occurrence of transition losses at the boundaries due to the adiabatic condition being satisfied. 
         [0079]    An indication of the cross-section of the second-stage fattened fibre  12   b  along line C-C is indicated at  28 . Typical dimensions of the expanded cross-section after up-tapering would be 45 μm diameter for the core  16  and 560 μm diameter for the cladding  18 . 
         [0080]    Considering an example of a stripped, two-stage fattened fibre, the minimum initial strip length would be approximately 160 mm, and governed by:
       A 5 mm clearance from the stripped buffers  14  at both ends of the processed fibre;   A centred first-stage fattened section length of 15 mm (to include the second-stage fattened section) up to a diameter of 280 microns; and   A centred second-stage fattened section length of 5 mm up to a diameter of 560 microns.       
 
         [0084]    It will be understood that further fattening stages may be performed as required, in order to further increase the diameter of a fattened fibre, provided that the conditions for fibre fattening are satisfied, e.g. the adiabatic condition. It is predicted that, given the conditions and limits for 2-stage fattening, a 3-stage fattening process may provide a 6-8 times increase in physical diameter from the original fibre size. Preferably, a minimum distance of approximately 5 mm is maintained between successive transition sections. 
         [0085]    Up-tapered fibres can then be used in the construction of different fibre optic devices. For example, for an active device, such as an erbium-doped laser, the fattened section provides an interface between a standard fibre and a separate, large mode area, erbium-doped fibre section. For a fibre spectrometer, a cleaved fattened end-face may be processed, for example, by inscribing a pattern, or photo-inducing a pattern, on the end-face using a laser. 
         [0086]    Taking the multi-stage fattened fibre  12   b  shown in  FIG. 3 , the fibre  12   b  is first cleaved across line C-C, i.e. across the widest cross-section of the fibre  12   b . Once a cleaved multi-stage fattened fibre  12   b  is provided, the fibre may be combined with a suitable section of a doped fibre in different manufacturing processes to produce improved optical devices employing multi-stage fattened fibres. Examples of such optical devices can be seen in  FIGS. 4(   a )-( c ). 
         [0087]    As the section of the doped fibre (indicated at  30  in  FIG. 4 ) is of a greater diameter than that of the original un-fattened optical fibre  12 , the mode-profile is flatter than for an un-fattened fibre with a Gaussian shape mode. A more efficient and more even interaction with the dopants in the fattened optical fibre  30  can therefore be provided in the fibre  30  than for the same length of the original, unfattened fibre  12  that results in a flatter gain response. 
         [0088]    Furthermore, the use of particular glass fibres, e.g. a phosphate glass fibre, can allow for a much higher concentration of dopant than, for example, silica glass fibre. Therefore, by selecting a first glass fibre appropriate for the multi-stage fattening (e.g. silica), and splicing the fattened fibre with a glass fibre suitable for high-concentration doping (e.g. phosphate), then an optical device can be manufactured having improved amplification for a relatively small footprint of device. 
         [0089]    It will be understood that, while the devices described employ phosphate glass fibres, other types of glass fibres may be used, e.g. phosphate, silica, telluride, fluoride, chalcogenide, bismuth. Similarly, while the examples utilise erbium as the dopant, the fibre section may be doped with any one of the following dopants: erbium, thulium, chromium, ytterbium, praseodymium, neodymium, terbium or a combination thereof. 
         [0090]    With reference to  FIG. 4(   a ), an optical wavelength converter is indicated generally at  32 . The converter  32  comprises a cleaved end  34   a  of the fattened fibre  12   b  coupled with the first end  30   a  of a length of fibre  30  having an erbium-doped phosphate core  36  with a suitable cladding  38 . The fibre  30  has a diameter substantially equal to the diameter of the cleaved end  34   a  of the fibre  12   b . The uncleaved, unfattened end of the fibre  12   b  is coupled with an optical buffer  14 , such that the buffer  14  and the fibre  12   b  together form a fattened fibre input launch for the optical wavelength converter  32 . The free second end  30   b  of the fibre section  30  can be used to provide a wavelength-converted output for a signal input at the optical buffer  14 . Such a converter  32  can be used for to provide wavelength conversion having high gain for various purposes, for example, to convert infrared light to visible light for the purposes of charge-coupled device (CCD) detection. 
         [0091]    With reference to  FIG. 4(   b ), an optical fibre amplifier is indicated generally at  40 . The amplifier  40  comprises a first cleaved end  34   a  of the fattened fibre  12   b  coupled with the first end  30   a  of a length of fibre  30  having an erbium-doped phosphate core  36  with a suitable cladding  38 . The second end  30   b  of the fibre  30  is coupled with a second cleaved end  34   b  of the fattened fibre  12   b.    
         [0092]    The respective first and second uncleaved, unfattened ends of the fibre  12   b  are indicated at  35   a , 35   b  in  FIG. 4(   b ). The first cleaved and uncleaved ends  34   a , 35   a  of the fibre  12   b  form a fattened fibre input signal lead and forward pump launch for the optical amplifier  40 . Similarly, the second cleaved and uncleaved ends  34   b , 35   b  form a fattened fibre output signal lead and reverse pump launch for the optical amplifier  40 . The optical amplifier  40  shown is suitable for use in systems to amplify optical signals as they propagate along relatively long fibres, e.g. for regeneration purposes. 
         [0093]    With reference to  FIG. 4(   c ), an optical fibre laser is indicated generally at  50 . The laser  50  may be constructed in a similar fashion to the optical amplifier  40  described above, with the distinction that, prior to the coupling of the cleaved ends  34   a , 34   b  of the fattened fibre  12   b  with the fibre section  30 , the cleaved ends  34   a , 34   b  are processed to provide partial end-reflections at the cleaved ends  34   a , 34   b . For lasing to occur in a fibre, a resonant gain cavity is required in the fibre. In general, some form of reflectors are provided at either ends of the resonant gain cavity, the cavity then being excited by a pump source. Resonance is achieved in the cavity by using reflectors operable to reflect light at the lasing wavelength within the cavity. In a fibre system, the end reflectors are typically fibre Bragg gratings—the most convenient and effective and preferred method. However, further types of reflecting means may be employed, for example, it would be possible to “drill” into the fibre either side of the lasing cavity and deposit silver/gold to achieve end reflection. In  FIG. 4(   c ), the reflectors are provided in the form of fibre gratings  52 . The gratings  52  provide for the partial reflection of light of a particular wavelength along the fibre section  30 . This further processing stage may involve any suitable grating manufacturing process, e.g. inscribing a grating pattern on the fibre, photo-inducing a grating pattern, etc. 
         [0094]    The invention is not limited to the embodiments described herein but can be amended or modified without departing from the scope of the present invention.