Abstract:
An optical fibre for use in fibre lasers has the lasing additive eg Er 3+ , concentrated in center of the core. Preferably the core has an inner region which contains the additive and an outer region which is dopant free. The concentration of the dopant reduces the pump threshold for a laser and improves the gain performance for a given pump power. The fibre is conveniently made in MCVD. The use of Al 2  O 3  in the inner zone appears to reduce loss of dopant during tube collapse.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical fibre with a fluorescent additive and in particular to fibre which is suitable for the construction of fibre lasers. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     There is currently much technical interest in a wide range of devices in which radiation at wave lengths of 0.3 μm to 4 μm is generated in the core of an optical fibre. In these devices the fibre contains a fluorescent additive which interacts with excitation radiation, usually identified as the &#34;pump radiation&#34;, to produce the desired output. The devices take many forms, e.g. broadband sources, super luminescent sources and temperature sensors, but devices which display laser activity are particularly important, especially in telecommunications. It should be realised that telecommunications uses laser activity in two distinct manners namely optical oscillators and optical amplifiers. However the same doped glass fibres are equally suitable for a plurality of such applications (and often for all such applications). 
     Stone and Nurrus in &#34;Applied Physics Letters&#34; (Volume 23, No. 7, 1 October 1973 at pages 388 and 389) disclose lasers made of neodymium doped silica with end-pumped fibre geometry. One of their systems has an active core of fused SiO 2  and Nd 2  O 3  enclosed in a thin passive sleeve of SiO 2  and Nb 2  O 5  all enclosed in a fused jacket of SiO 2 . The diameter of the active core was about 800 to 15 μm and the length of the samples was 1 cm. The function of the thin passive sleeve is to increase the guidance of the core and hence pump efficiency. 
     U.S. Pat. No. 3 808 549 describes an optical waveguide light source having an active core surrounded by an inner cladding layer and an outer cladding layer. The refractive index of the outer cladding is lower than the refractive index of the inner cladding which is lower than the refractive index of the core. Pump radiation is launched into the inner cladding layer to which it is confined by the outer cladding. The pump radiation makes many passes through the core whereby its absorption by the core is increased. The signal is generated within the core. 
     It has long been recognised that the rare earth elements, e.g. Nd, Er, and Yb, display fluorescent properties which make them suitable for use as fluorescent additives in optical fibre. Their fluorescent properties make them particularly suitable for use in the laser devices mentioned above. The operation of a fluorescent device clearly depends on absorption of the pump photons in order to raise ions (or other fundamental particles) of dopant to an excited state for subsequent emission on a different transition. In a laser device, this emission is stimulated by the presence of a signal photon and, therefore, the operation of a laser device also depends on the interaction of radiation at signal wave length. It is an object of this invention to make efficient use of pump power launched into optical fibre. In the case of optical amplifiers this means achieving high gain for small launched pump powers whereas for optical oscillators it implies a low lasing threshold. 
     Fibre according the invention has a fluorescent additive unevenly distributed over the cross section of the core and having a higher concentration of the additive at the centre of the core than at the core/cladding boundary. The highest concentrations of additive should ideally be located in those regions of the fibre where, during pumping, the highest intensity of pump radiation is to be expected. Lower or zero concentrations of the additive should be located where only low pump intensities are to be expected. 
     In most pumping schemes the highest intensity of the pump radiation will be located at the centre of the core and it is appropriate to provide the highest dopant concentration at the centre of the core with zero concentration at its periphery. Preferably the core comprises two zones, namely an inner zone surrounded by an outer zone wherein the inner zone contains the dopant and the outer zone contains substantially no dopant. Suitably the inner zone constitutes less than a quarter, e.g. 5 to 15%, of the cross sectional area of the core. 
     The fibre may be implemented in any glass system which is compatible with the fluorescent dopants. Thus, for example, the fibre may be implemented in conventional silicate, phosphate and fluoride systems, eg. fluorides of Zr, Ba, La, Al, Na and Hf or in silica systems, eg. SiO 2  with additives such as GeO 2  to adjust the refractive index in the core. 
     In a specific embodiment silica fibre has: 
     (a) a cladding formed of SiO 2  with P 2  O 5  to reduce the melting point, and F to offset the increase in refractive index, 
     (b) an outer core region formed of SiO 2  with GeO 2  to increased the refractive index and P 2  O 5  to reduce the melting point; and 
     (c) an inner core region formed of SiO 2  with Al 2  O 3  to increase the refractive index, P 2  O 5  to decrease the melting point and prevent devitrification and a fluorescent dopant to interact with the pump radiation. 
     The dimensions of the fibre are preferably such that it is single mode at signal wave length. This implies that it may be able to support several, e.g. up to 4 or 5, modes at pump frequency. The fluorescent dopants of major interest include the rare earth metals. Of these the most important are Er (which lases by the three level mechanism) and Nd (which lases by the 3 and four level mechanism) 
     One method of making silica fibre according to the invention utilises the modified chemical vapour deposition process usually identified as MCVD. MCVD is sometimes known as the inside deposition process because the glasses which eventually form the operative parts of the fibre are produced by converting the equivalent chlorides into the desired oxides which are deposited, layer by layer, on the inner surface of a substrate tube. Usually a total of 10 to 30 layers are deposited. As initially deposited the glass is porous but the porous material is immediately fused to give a solid layer upon which subsequent layers are deposited. When all the layers have been deposited the tube is collapsed to a rod which is drawn into fibre. 
     To make fibre according to the invention this procedure is followed for the cladding and the outer core. The precursor of the inner core is deposited but left in the porous state. Dopants, including the fluorescent additive, are introduced as solution into the porous layer. After solvent removal and conversion to oxides as necessary, the porous layer is consolidated and the tubular configuration is collaped into a rod which is then drawn into fibre. 
     It will be appreciated that this, i.e. soaking a solution into a porous layer, is one of many known techniques of introducing dopants into optical fibre. It has been adapted, in accordance with the invention, to provide a small, doped centre region in a larger core. One difficulty inherent in MCVD is that there is a tendency to lose dopant by evaporation from the exposed inner surface. This is not acceptable since the invention requires a high concentration of dopant at the axis. The depleted zone can be removed, e.g. by etching, just before final collapse. Although there appears to be a risk that further loses could occur during the final stage of the collapse, this does not happen to any noticeable extent because: 
     (1) The exposed surface is so small that the rate of loss is minimal. 
     (2) The final stage only takes a time which is too brief for noticeable loss to occur. 
     However we have most surprisingly discovered that, when aluminium is used to adjust the refactive index of the core, the loses of fluorescent dopant are not noticeable. The aluminium can be introduced at the same time as the fluorescent dopant, e.g. as Al(NO 3 ) 3  in alcoholic solution. During heating the Al(NO 3 ) 3  is converted to Al 2  O 3 . 
     The fibre according to the invention can be used to make conventional fibre devices which include a pump for providing excitation radiation for the fluorescent additive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying drawing which is a cross section through a fibre according to the invention; 
    
    
     DETAILED DESCRIPTION 
     The drawing shows a fibre according to the invention prepared by the MCVD process. This fibre has a residual layer 10 which is the residue of the substrate tube used in the MCVD process. The layer 10 has no optical function. The fibre also has a conventional cladding 11, a core which is generally indicated by the numeral 14, having an (undoped) outer region 12 an inner region 13 which contains a fluorescent dopant, e.g. Er, at a concentration of 0.001-10.0 wt % Er. 
     Fibre as described above was prepared by a substantially conventional MCVD process in which a substate tube was rotated in a glass blowing lathe while a reactant gas was passed through its bore. In the course of the preparation three different reactant mixtures, to be specified below, were used. 
     A short segment of tube, about 2 cm long, was heated to reaction temperature by a travelling flame. In this segment chlorides were converted into oxides which deposited as a porous ring downstream of the flame. As the flame traversed, in the case of cladding and outer core, the deposit was fused to form a thin layer on the inner surface of the substrate tube. In the case of inner core, a lower temperature was used so that the deposit did not fuse whereby it remained porous. 
     The reaction mixture used to form the cladding precursor was obtained by bubbling: 
     700 ml/min of O 2  through SiCl 4 , 
     90 ml/min of O 2  through POCl 3 . 
     The mixture of these two gas streams was diluted with 1.5 liter/min O 2  and 1.0 liter/min He. In addition, 6 ml/min of CCl 2  F 2  were included in the mixture. The maximum temperature in the hot zone was 1600° C. and the flame travelled at 18 cm/min. 
     Five cladding layers were thus deposited on a substrate tube of 10 mm inner diameter. These fused together to form a cladding layer of SiO 2  doped with P 2  O 5  and fluorine. 
     (The P 2  O 5 , which is derived from the POCl 3 , is incorporated to reduce the melting point of the SiO 2  which makes the fusion easier. The P 2  O 5  slightly increases the refractive index of the silica but the fluorine slightly reduces the refractive index. By balancing the two concentrations the refractive index of the five cladding layers is sunstantially equal to that of pure silica. Thus the POCl 3  and C Cl 2  F 2  are processing aids which have little or no effect on the performance of the final product which, therefore, consists essentially of SiO 2 ). 
     Eight layers to form the outer core were deposited next. The reaction mixture used for each layer was obtained by bubbling: 
     200 ml/min at O 2  through SiCl 4   
     200 ml/min of O 2  through GeCl 4   
     10 ml/min of O 2  through POCl 3 . 
     The mixture of these three gas streams was diluted with 1.5 liters/min of O 2 . These eight layers were fused together at 1500° C. and the flame travelled at 16 cm/min. This formed the outer core region which consisted essentially of SiO 2  doped with GeO 2  to increase the refractive index and P 2  P 5  to facilitate processing by lowering the melting point of the glass. 
     The precursor at the inner core was deposited in two porous layers. The reaction mixture was obtained by bubbling: 
     200 ml/min of O 2  through SiCl 4   
     10 ml/min of O 2  through POCl 3  and diluting with 1.5 liters/min of O 2 . The torch traverse rate was 17 cm/min and the maximum temperature was at 1300° C. (which is too low to fuse the deposit). 
     (Note. In all bubbling operations the liquid was at 25° C.) At this point the tube was removed from the lathe and dopants were introduced into the porous layers by immersion for 1 hour in an ethanolic solution of 
     1M Al(NO 3 ) 3   
     0.08M ErCl 3 . 
     After soaking, the tube was drained, blown dry with N 2  for one hour, and returned to the lathe where it was heated at about 800°-1000° C. for a few minutes. This completed solvent evaporation. The temperature was raised to about 1900° C. for collapse to a rod. This also ensured conversion of Al(NO 3 ) 3  to Al 2  O 3  and ErCl 3  to Er 2  O 3 . The tube was flushed with O 2  /He mixture during all of these stages (about 10% (volume) of Cl 2  could be introduced into the O 2  /He mix if a very dry product were required.) 
     The resulting perform had a core about 2 mm diameter. Analysis (using disperive X-ray techniques) confirmed that Al and Er 3+  were confined to the central region. The reason for choosing a large core will be briefly explained. 
     The ultimate objective is a fibre having the dopant, Er 3+ , contained in a very small inner core, e.g. with a diameter of 1 to 3 μm. It was decided to achieve this by means of a fat outer core (8 layers), a thin inner core (2 layers) and a high overall draw ratio, i.e. length extension of about 1:10 5 . 
     The fatness of the preform made it difficult to handle and it was drawn in two stages. First the external diameter was reduced from 7.0 mm to 3.2 mm, i.e. an axial draw of 1:4.8. The drawn rod was sleeved with a silica tube and then drawn 1: 2.5×10 4  to give the product fibre. 
     (There is a well known problem that dopants are lost during collapse from the inner layers of the preform. This results in a thin axial depletion zone. In the process above described Al 2  O 3  was present and in the presence of this compound no loss of Er 3  + occurred.) The product fibre had the following measurements. 
     
         ______________________________________  Cladding (11)  OD      7 μm  ID      4 μm  RI      matched to silica  Core    Outer Region 12  OD      4 μm  ID      1.5 μm  Er.sup.3+          NONE  Core    Inner Region 13  Diameter          1.5 μm  Er.sup.3+          1 wt %  General Properties  OD                 125 μm  LP.sub.11          Cut Off    790 nm  RI step            0.01______________________________________ 
    
     &#34;RI step&#34; denotes the difference between the RI of the core and the RI of the cladding. 
     A possible theory of the operation of this fibre will now be briefly indicated. 
     The considerations set out above are particularly pertinant to dopants which lase as a three level system. The three levels are: 
     (a) The ground state, 
     (b) the pump level, 
     (c) upper laser level, (also known as the metastable level). 
     The absorption of pump photon by a ground state ion transfers that ion to the upper pump level from where it decays non-radiatively to the upper laser level. That ion can then return to the ground state via the lasing transition, i.e. gives out a signal photon. In order to achieve the population inversion essential for laser operation it is necessary to pump more than half the dopant ions from the ground state to the upper laser level. Thus it is important to note that, at a particular point in space, if fewer than half the ions have been pumped to the upper laser level then the signal beam will be attenuated at that point. 
     It is therefore extremely desirable to preferentially locate the fluoresence additive where the pump intensity is highest, i.e. on the axis and to prevent there being any dopant ions in the regions where the pump intensity is lower. 
     The signal beam, which being single mode also has its maximum intensity on the axis, overlaps well with the excited dopant ions and thus effectively depopulates the upper laser level. 
     In order to illustrate the benefit of the invention comparative measurements were made on two very similar fibres both of which used Er 3+  ions as the flurescent species. The fibre, identified as &#34;A&#34;, had the Er 3+  ions located in a centre core Region 13 as shown in FIG. 1. The comparative fibre, identified as &#34;X&#34;, had the more standard Er 3+  distribution, i.e. uniformly distributed over the whole of the core. Details of both fibres are given in Table 1. 
     
         ______________________________________Diameters (μm)          A            X______________________________________Total          125          125To Cladding (11)          6.0          6.0To Core (12)   3.5          3.5Inner Region (13)          1.0          NONERI Step        0.0100 ± 0.005                       0.0095 ± 0.005LP.sub.11 Mode cut off (nm)          790          790______________________________________ 
    
     In the case of fibre A the dopant was contained only in the inner region 13. Based on this region alone the concentration of Er 3+  was 0.45% wt or 0.037% wt based on the total core 14. For fibre X the concentration of Er 3+  was 0.05% wt based on the total core 14. 
     The performances of the two fibres were compared by measuring the &#34;transparency power&#34; of each. 
     To measure the transparency power a short length of fibre is used so that pump power does not change significantly along the length. The test comprises launching the signal at wave length 1.54 μm and pump at wave length 528.7 nm into the opposite ends of the fibre. The input and output powers of the signal are measured for several values of pump power. There exists a specific pump power at which the signal is neither amplified or attenuated and this power is known as the &#34;transparency power&#34;. This name is considered appropriate because, at this pump power, the fibre simulates a perfectly transparent window. At higher pump powers than the transparency power, the fibre amplifies the signal beam whereas at lower pump powers the fibre attenuates the signal beam. The transparency power is a direct measure of the performance of the invention and the lower the transparency power the better the performance. The transparency powers of the two fibres was 
     
         ______________________________________  Fibre A       0.8 mW  Fibre X       1.4 mW  Ratio         1:1.75______________________________________ 
    
     Thus the fibre according to the invention gave gain at much lower pump power than the comparative fibre.