Patent Publication Number: US-2006001952-A1

Title: Method and device for producing optical material, and an optical waveguide

Description:
The present invention relates to a method for producing light-amplifying optical material, said method comprising at least atomizing at least one reactant in liquid form by an atomizing gas to form droplets, introducing said droplets and/or their vaporous products into a flame, oxidizing said at least one reactant to form one or more, condensing said one or more oxides to produce particles, collecting at least a part of said particles, and fusing said particles together to form said light-amplifying optical material. The present invention relates also to a device for producing said light-amplifying optical material and to an optical waveguide comprising said light-amplifying optical material.  
     BACKGROUND OF THE INVENTION  
      Generation of small particles is an important step in the production of light-amplifying optical waveguides, which amplify light by stimulated emission of radiation. The light-amplifying properties of those waveguides are achieved by doping, for example, amorphous quartz glass with suitable dopants, for example with erbium.  
      Doped quartz glass can be produced by generating small particles by synthesis in a flame. U.S. Pat. No. 6,565,823 discloses a method and an apparatus for forming fused silica by combustion of liquid reactants. Liquid siloxane feedstock is delivered as a liquid solution to a conversion site, which may be, for example, a methane-oxygen flame. The feedstock is atomized with the assistance of a gas to form a dispersion of liquid droplets. The droplets are evaporated and the siloxane is decomposed and oxidized in the flame to form supersaturated silica vapor. The saturated vapor pressure of silica is low even at the high temperatures of the flame. Consequently, the supersaturated vapor is rapidly nucleated and condensed generating a number of small silica particles. The particles are collected on a mandrel to form a waveguide preform. A waveguide is subsequently produced from the preform by a process comprising heating and drawing.  
      Due to differences in the saturated vapor pressures of required reactants, it may be advantageous to introduce reactants with low saturated vapor pressure into the flame as atomized liquid droplets. The process of forming small droplets by aerodynamic and/or shear forces caused by a gas stream acting on a liquid surface is called atomization.  
      It is a known fact in the area of atomization, that small liquid droplets may be produced using a high velocity of the atomizing gas. However, it is known in the area of producing light-amplifying materials, that it is critical for the optical and mechanical properties of the material, that the properties of the produced material particles are as homogeneous as possible. Consequently, particles are typically synthetized in a flame, which does not exhibit large spatial and temporal variations of temperature and local gas composition. Therefore the tendency has been to minimize the turbulence of the flame in order to achieve a reaction zone, which is spatially and temporally uniform and preferably laminar. It is known that high gas velocities induce turbulence, which in turn is associated with chaotic spatial and temporal variations of temperature and local gas composition. The requirement to obtain well-controlled uniform properties of the flame has set a limit to the velocity of the atomizing gas.  
      Another aspect is that a long residence time in the flame is known to favor complete evaporation of the droplets and to ensure reaction times, which are long enough for oxidation and the formation of desired compounds. It is known that the residence times are proportional to the length of the flame and inversely proportional to the velocity of the gas or the droplets.  
      U.S. Pat. No. 6,565,823 teaches that in a most preferred embodiment high velocity gas is utilized in atomizing a liquid feedstock, which gas produces atomized liquid projections with a velocity in the range of 0,5 to 50 m/s. Further, using a gas flow rate and minimum diameter values indicated on column 10, lines 1 to 11 of said patent, a velocity in the order of 50 m/s can be calculated for said atomizing high velocity gas.  
      Patent application PCT/FI99/00818 teaches in a similar fashion, that for effective atomization, it is preferable to make the velocity of the spraying gas as high as possible. However, no numerical values are given for said velocity.  
      U.S. Pat. No. 6,672,106 discloses a modification of the system described in the U.S. Pat. No. 6,565,823. The U.S. Pat. No. 6,672,106 teaches that by using said modification and by using oxygen as the atomizing gas, the velocity of the atomizing gas stream can be reduced by at least 50%. In addition, the U.S. Pat. No. 6,672,106 teaches that by using lower atomizing gas velocities, turbulence is reduced at the reaction zone, and thus the particle deposition rate is greatly improved. A reduction in gas velocity is also taught to reduce so called blank defects, which are detrimental to the optical and mechanical properties of the produced waveguides.  
     BRIEF DESCRIPTION OF THE INVENTION  
      It is an object of the present invention to produce light-amplifying optical material with homogenous composition and small size. It is a further object of the present invention to achieve improved control of the process used in the production of said material.  
      To attain these objects, the method and the device according to the present invention is mainly characterized in that atomizing gas atomizing a reactant in liquid form is discharged at a velocity, which is in the range of 0.3 to 1.5 times the velocity of sound. The light-amplifying optical waveguide according to the present invention is mainly characterized in that atomizing gas is discharged at a velocity, which is in the range of 0.3 to 1.5 times the velocity of sound, and that the concentration of clustered erbium ions in produced light-amplifying optical waveguide material is smaller than the square of the concentration of all erbium ions in said light-amplifying optical waveguide multiplied by a factor 6×10 −27  m 3 . Other preferred embodiments of the invention are described in the dependent claims.  
      According to the present invention, homogeneous particles suitable for producing optical waveguides are achieved by maximizing turbulence in the flame. Thus, the approach according to the present invention is different from the approach used in the prior art.  
      The flame becomes highly turbulent and the rates of mixing, heating and cooling are greatly enhanced. Thanks to efficient mixing, the generation of heat, the reactions and the condensation of the particles take place fast and essentially in the same volume within the flame, which improves the control of the particle production process.  
      By applying a high velocity of the atomizing gas several advantageous effects take place: The average size of the atomized droplets becomes small thanks to the high velocity of the atomizing gas. The atomized droplets are rapidly transferred to the flame. The high velocity of the atomizing gas enhances turbulence and mixing of the reactants in the flame. Thanks to effective mixing the reaction rates are high. The high rate of combustion leads to high combustion temperature, which further accelerates the rates of oxidation and doping reactions and accelerates gas velocity in the flame. Thanks to the high temperature and small droplet size, the droplets are evaporated rapidly in the flame. The dimensions of the flame are shrunk thanks to the high reaction rates. Turbulence enhances also mixing of cold gas to the reaction gases reducing the effective residence times even further. Thanks to the high gas velocity and small dimensions, the residence time of the substances in the flame are reduced. The low residence times reduce the agglomeration of the droplets and the produced particles.  
      The turbulent flame is not sensitive to disturbances. Therefore the production capacity of the device and the method according to the present invention can be scaled up by arranging several devices to operate adjacent to each other.  
      The residence time of the reaction products in the flame is short. Thus particles comprising nonequilibrium chemical products can be produced. For example, the separation of different phases in the produced material and the undesired clustering of erbium ions are minimized, which improves the homogeneity of the produced particles.  
      This is advantageous especially in the production of particles suitable for manufacturing of light-amplifying optical waveguides. For example, in case of doping with erbium, the aim is to have single and isolated erbium ions in the material. Clustered forms of erbium are not effective in the amplification of light. Erbium has a tendency to form Er 2 O 3  in the gas phase, if sufficient time is available to reach thermodynamical equilibrium. In an Al—Si—O system erbium has a tendency to form Al 5 Er 3 O 12 .Al 2 O 3 , respectively. According to the invention, the formation of the erbium ion clusters can be minimized by limiting the residence time of the particles in the flame, which is achieved by applying the high velocity of the atomizing gas.  
      Because the clustering of the active ions is substantially minimized, it is possible to increase the concentration of said ions in the produced light-amplifying material, which consequently leads to high quantum conversion efficiency. Thus, erbium-doped optical waveguide produced according to the present invention has excellent light-amplifying characteristics. For example, an Er-doped fiber produced according to the present invention was found to provide a quantum conversion efficiency of 65%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1   a  shows a schematic side cross-sectional view of the burner assembly in accordance with the present invention,  
       FIG. 1   b  shows an schematic axial view of the burner assembly of  FIG. 1   a,    
       FIG. 2  is a schematic representation of the production and collection of particles in accordance with the present invention,  
       FIG. 3  shows a preferred embodiment of the burner assembly in accordance with the present invention,  
       FIG. 4   a  is a schematic representation of a device for producing an optical waveguide preform,  
       FIG. 4   b  is a schematic representation of drawing an optical waveguide from an optical waveguide preform,  
       FIG. 5  is a flow chart of the production of an optical waveguide in accordance with the present invention,  
       FIG. 6  shows a schematic side cross-sectional view of a further embodiment of a burner assembly with an annular Laval nozzle,  
       FIG. 7   a  shows a schematic side cross-sectional view of a further embodiment of a burner assembly with a Laval nozzle and with two transverse liquid nozzles,  
       FIG. 7   b  shows a schematic axial view of the burner assembly of  FIG. 7   a,    
       FIG. 8   a  shows a schematic side cross-sectional view of a further embodiment of a burner assembly with a plurality of liquid nozzles,  
       FIG. 8   b  shows a schematic axial view of the burner assembly of  FIG. 8   a,    
       FIG. 9  shows a schematic side cross-sectional view of a further embodiment of a burner assembly with a further diverging nozzle, and  
       FIG. 10  shows a further embodiment of a burner assembly with a swirl-inducing element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The device for making light-amplifying optical material comprises at least a burner assembly, which is used for producing particles of erbium-doped silica glass.  
      Referring to  FIGS. 1   a  and  1   b,  the burner assembly  600  comprises four tubes  11 ,  21 ,  31 ,  41 , which define four concentric nozzles  12 ,  22 ,  32 ,  42 . The innermost nozzle, herein called as the liquid nozzle, is used for delivering liquid reactant  10 . The outer surface of the tube  11  and the inner surface of the tube  21  define together an annular atomizing gas nozzle  22 , from which an atomizing gas  20  is discharged. The atomizing gas is accelerated by a pressure difference prevailing over the nozzle  22 . The velocity of the atomizing gas  20  may be further accelerated by the constriction  24  of the nozzle  22 . Instead of the constriction  24  of the tube  21 , the cross-section may also be reduced by implementing an enlargement of the outer surface of the liquid reactant tube  11 . The burner assembly  600  may also comprise more nozzles than depicted in  FIG. 1   a,  for example to deliver inert gas.  
      Referring to  FIG. 2 , there is a liquid surface  14  at the liquid nozzle  12 . Shear and aerodynamic forces generated by the stream of the atomizing gas  20  tear droplets  15  from the liquid surface  14  causing atomization. The droplets may be further fragmented by turbulence. The droplets are entrained within the gas jet and accelerated to a high velocity and further entrained into the flame  100 .  
      The reactants delivered by the nozzles  12 ,  22 ,  32 ,  42  are mixed by turbulence and by diffusion. Exothermic reactions of the reactants, especially the oxidation of hydrogen provides the heat required for the flame  100 . A high temperature is achieved. The origin of the flame  100  is associated with a position in which the velocity of the flame propagation with respect to the gases is substantially equal to the velocity of the gases.  
      Due to extensive dilution with surrounding gases, the atomized droplets  15  start to evaporate after atomization. The rate of evaporation is greatly enhanced after mixing with the hot combustion gases in the flame  100 . The reactants  10 ,  20 ,  30  react and oxidize in the flame  100  by producing oxides and other compounds. The saturated vapor pressures of silicon oxide (silica) and erbium oxides are so low that they are rapidly nucleated and condensed forming doped silica particles  50 . The condensation is further promoted by the turbulent mixing of surrounding cool gas with the hot reaction gases, which rapidly decreases the average temperature of the gases.  
      Preferably, the velocity of the atomizing gas  20  near the liquid surface  14  is in the range of 0.3 to 1.5 times the velocity of sound. The most preferred velocity of the atomizing gas is substantially equal to the velocity of sound.  
      The velocity of sound V S  is given by the equation:  
                 V   S     =         p   ⁢           ⁢   γ     ρ         ,           (   1   )             
 
 in which p denotes gas pressure, ρ denotes gas density and the constant γ is given by:  
             γ   =         c   p       c   v       .             (   2   )             
 
 in which c p  denotes the heat capacity of the gas at constant pressure and c v  denotes the heat capacity of the gas at constant volume. The velocity of sound V S  depends on the gas temperature and on the type of the gas. 
 
      The Reynolds number Re D  corresponding to a velocity V is defined as:  
                 Re   D     =     VD   v       ,           (   3   )             
 
 in which D is the outer diameter of the liquid nozzle  12  and ν is the kinematic viscosity of the atomizing gas at the exit end of the atomizing gas nozzle  22 . 
 
      It is known that a high Reynolds number promotes turbulence. In order to achieve small droplets  15 , it is advantageous to select a small diameter of the liquid nozzle  12 , which according to the equation (3) requires a high velocity V of the atomizing gas  20 .  
      A pressure ratio R is defined as:  
               R   =       p   i       p   0         ,           (   4   )             
 
 in which p i  is the static pressure of the atomizing gas  20  inside the atomizing gas nozzle  22  and p o  is the static pressure outside the atomizing gas nozzle. It is known that a substantially sonic velocity, i.e. the velocity of sound may be reached when the pressure ratio R prevailing over the constriction  24  ( FIG. 1 ) has a value in the order of two. 
 
      Only velocities up to the velocity of sound can be implemented using nozzles with constant cross section or with converging cross-section. The implementation of the velocities higher than the velocity of sound requires diverging nozzles.  
      In order to produce erbium-doped silica material, the liquid reactant delivered by the nozzle  12  is preferably erbium chloride and aluminum chloride dissolved in methanol. The atomizing gas delivered by the atomizing gas nozzle  22  is hydrogen. Silicon tetrachloride is delivered by the annular nozzle  32  ( FIG. 1 ) and oxygen is delivered by the annular nozzle  42  ( FIG. 1 ). The role of aluminum chloride is to improve the solubility of erbium in the produced silica glass.  
      Further, the applicable flow rates in the production of erbium-doped silica are as follows:  
      Liquid flow rate through the liquid nozzle  12 : 3.6 to 4.5 g/min.  
      Gas flow rate through the atomizing gas nozzle  22 : 35 to 60 SLPM.  
      Gas flow rate through the nozzle  32 : 0 to 15 SLPM.  
      Gas flow rate through the nozzle  42 : 10 to 40 SLPM.  
      SLPM denotes standard liter per minute.  
      In the production of erbium-doped silica, the applicable diameters of the nozzles  12 ,  22  are substantially in the order of a millimeter.  
      The optimum combination of the flow rates of the reactants  10 ,  20 ,  30 ,  40 , the composition of the reactants  10 ,  20 ,  30 ,  40 , and the dimensions of the nozzles  12 ,  22 ,  32 ,  42  should be optimized according to the predetermined target properties of the light-amplifying optical material. For example, the predetermined target concentration of erbium ions may be set to correspond an absorption of 10 dB/m, 20 dB/m or a further predetermined value. The preferred approach is that the optimum flow rates, compositions and dimensions are determined by an experimental procedure known by a person skilled in the art. It is emphasized that a determined approach to apply an atomizing gas velocity in the order of the velocity of sound is required. Typically, a set of experiments has to be carried out, i.e. a single test using a high atomization gas velocity is not likely to provide the optimum parameters.  
      When producing erbium-doped silica material, the most preferred embodiment of a burner assembly  600  according to the present invention is shown in  FIG. 3 . The burner assembly  600  comprises a small-diameter tube  11  and machined tubular parts  21 ,  23 ,  31  and  41 , which define four concentric nozzles  12 ,  22 ,  32  and  42 . The outer surfaces of the tubular parts  21  and  31  comprise longitudinal grooves, which provide the centering of the components, while also allowing the flow of the gaseous reactants  30  and  40  through the nozzles  32  and  42 . The tubular part  23  comprises machined slots, which allow the flow of the atomizing gas  20  to a narrow annular gap, i.e. to the atomizing gas nozzle  22  defined by the inner surface of the tubular part  23  and the outer surface of the small-diameter tube  11 . Said gap acts also as a constriction, which accelerates the velocity of the atomizing gas. The liquid reactant  10  is delivered through the small-diameter tube  11 . The flows of the gaseous reactants  20 ,  30  and  40  are directed to the nozzles  22 ,  32  and  42  by a plurality of flanges  61 ,  62 ,  63 ,  64 .  
      The most preferred dimensions of the nozzles  12 ,  22 ,  32 ,  42  are as follows:  
      Inner diameter of the liquid nozzle  12 : 0.53 mm.  
      Outer diameter of the atomizing gas nozzle  22 : 1.0 mm.  
      Inner diameter of the atomizing gas nozzle  22 : 0.64 to 0.69 mm.  
      Outer diameter of the nozzle  32 : 4.0 mm.  
      Inner diameter of the nozzle  32 : 2.0 mm.  
      Outer diameter of the nozzle  42 : 6.0 mm.  
      Inner diameter of the nozzle  42 : 4.0 mm.  
      The most preferred flow rates are as follows:  
      Liquid flow rate through the liquid nozzle  12 : 4.5 g/min.  
      Gas flow rate through the atomizing gas nozzle  22 : 52 SLPM.  
      Gas flow rate through the nozzle  32 : 4 SLPM.  
      Gas flow rate through the nozzle  42 : 15 SLPM.  
      SLPM denotes standard liter per minute.  
      The most preferred compositions of the liquid and the gases are:  
      Liquid  10  through the liquid nozzle  12 : 85% by weight methanol+1.6% by weight metallic aluminum+0.05% by weight sodium chloride+0.36% by weight erbium chloride hexahydrate+water and acids to balance.  
      Gas  20  through the atomizing gas nozzle  22 : 100% hydrogen.  
      Gas  30  through the nozzle  32 : 11% by vol oxygen+89% by vol silicon tetrachloride.  
      Gas  40  through the nozzle  42 : 100% oxygen.  
      In general, in order to achieve desired light-amplifying properties of the end-product, the liquid reactant  10  may comprise a compound which may comprise at least one metal selected from the groups IA, IB IIA, IIB IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the periodic table of elements. Especially, the liquid reactant  10  may comprise erbium, yfterbium, neodymium and/or thulium. Silica-forming compounds may also be introduced in liquid form, for example by introducing siloxane. In some applications, one of the reactants may be clean room air. The atomizing gas  20  may be a premixed mixture of a combustible gas and an oxidizing gas, especially a premixed mixture of hydrogen and oxygen.  
      The flow rate of the liquid reactant  10  is controlled by a metering pump. The flow of the liquid reactant  10  may be partially assisted by a venturi effect generated by the atomizing gas stream  20 . The flow rates of the atomizing gas and the gaseous reactants  20 ,  30 ,  40  are controlled by thermal mass flow controllers. Silicon tetrachloride is introduced to the reactant  30  using a gas bubbler.  
      Referring to  FIG. 4   a,  a device  1000  for producing optical waveguide preform comprises a burner assembly  600 , a rotating mandrel  710  and a manipulator  800  to rotate and move the mandrel  710  with respect to the burner assembly  600 . The doped glass particles are synthetized in the flame  100  and collected on the mandrel  710  to form a preform  720 . Also additional glass material may be collected on the preform to provide material for the cladding of optical waveguide to be produced.  
      The mandrel is removed, and the preform  710  is subsequently inserted into a furnace (not shown) for purification and sintering. Referring to  FIG. 4   b,  the preform is finally heated and drawn to form an optical waveguide  750 , using methods and devices known by a person skilled in the art of optical fiber production.  
      At least a light-amplifying optical fiber with the following parameters can be produced by a method according to the present invention:  
      Peak absorption 20 dB/m measured at the wavelength of 1530 nm.  
      Core diameter  6  micrometers and cladding diameter 125 micrometers.  
      The percentage of erbium ions in clusters in the core material being in the order of 6.5%  
      The percentage of erbium ions in clusters can be determined on the basis of the ratio of the spectral transmittance of the optical material measured using a high intensity light source and the spectral transmittance of the optical material measured using a low intensity light source. The measurements are made at the wavelength of 978 nm.  
      The concentration of clustered erbium ions can also be expressed in a more general way. The percentage of erbium ions in clusters has been found to depend on the concentration of all erbium ions in the produced light-amplifying material. It has been experimentally found, that the percentage of erbium clusters in the light-amplifying optical material produced according to the present invention is typically equal or smaller than the concentration of erbium ions times a factor 4.85×10 −25  m 3 . Thus, allowing a typical error margin of 20%, the obtainable concentration of clustered erbium ions in produced light-amplifying optical waveguide material is smaller than the square of the concentration of all erbium ions in said light-amplifying optical waveguide multiplied by a factor 6×10 −27  m 3 .  
      During the operation, either the substrate  200  and/or the burner assembly  600  may be moved in linear, curved or rotational manner to collect the produced particles  50 . The collection of the produced particles  50  is mainly based on thermoforesis. However, also the principles of inertial impaction or collection by electrostatic forces may be applied for collecting the produced particles  50 . The device may be contained within an enclosure to maintain high purity of the generated product.  
      The device  1000  may also be used to produce and collect light-amplifying material on a planar surface, such as the substrate  200  shown in  FIG. 2  to form a planar, i.e. a substantially two-dimensional waveguide structure.  
      A plurality of tubes and or longitudinal rods comprising light-amplifying material may be arranged adjacent next to each other to be heated and drawn to form a so-called photonic optical structure.  
      An optical component comprising said light-amplifying material may be produced. For example, a light-amplifying rod may be produced by fusing, grinding and polishing processes to be used as a mounted or freestanding component in a laser device.  
       FIG. 5  is a flow chart of the method according to the present invention. The liquid reactant  10  is atomized  410  to droplets  50  in an atomizing step  410  using the atomizing gas  20 . The droplets  50  experience evaporation in an evaporation step  420  in the flame  100  and also prior to the introduction into the flame  100  ( FIG. 2 ). The evaporation products, the other gaseous reactants  30  and the oxidizing gas  40  is mixed to the gases causing oxidation in an oxidizing step  440 . Doping reactions take place in a doping reaction step  450 . Oxidation liberates heat  110 , which sustains the temperature of the flame  100  ( FIG. 2 ) and assists the evaporation of the droplets  50 , the oxidizing reactions and the doping reactions  450 . Supersaturated gas phase oxides are formed, which are rapidly nucleated and condensed to particles in a condensation step  460 . External cooling gas  120  may be allowed to mix with the hot reaction gases to further promote condensation in a further condensation step  470 . The produced particles  50  are separated from gases in a separation step  480  and collected on the substrate  200  in a collection step  490 . The separation step  480  and the collection step  490  take place primarily by thermophoresis.  
      It is emphasized, that thanks to the efficient mixing in the flame  100  ( FIG. 2 ), especially the oxidation step  440 , the doping reactions step  450  and the condensation steps  460 ,  470  take place at a very fast rate and within a small volume of the flame. Consequently, the residence times of the reactants  10 ,  20 ,  30 ,  40 , the reaction products and the particles  50  within the flame are so short, that the reactions leading to clustering of erbium ions and the reactions leading to the separation of the different phases of the doped silica glass do not reach equilibrium. As pointed out before, this is especially beneficial regarding the light-amplifying properties of the produced doped silica.  
      Referring to a further embodiment shown in  FIG. 6 , the velocity of the atomizing gas jet may be further increased by implementing an annular atomizing gas nozzle  22 , which has a diverging cross section, for example a portion with a conically expanding inner surface. Such a nozzle may comprise also a constricted section  24 . Preferably the nozzle  22  has the form of a Laval nozzle, which form is shown in  FIG. 6 . It is known that gas can be accelerated to a supersonic velocity using a Laval nozzle. Supersonic means a velocity, which is higher than the velocity of sound. So-called shock waves often exist in supersonic flows. The origin of the flame  100  ( FIG. 2 ), i.e. the boundary of the flame near the nozzles may be stabilized to a position, which coincides with the position of a shock wave.  
       FIGS. 7   a  and  7   b  show a further embodiment having one or more liquid nozzles  12  arranged according to a perpendicular geometry with respect to the atomizing gas nozzle  22 .  
       FIGS. 8   a  and  8   b  show a further embodiment having several liquid nozzles  12  arranged within one atomizing gas nozzle  22 . One or more nozzles supplying gaseous reactants may also be arranged within the atomizing gas nozzle  22 . This kind of a set-up is advantageous when scaling up the device  1000  according to the present invention.  
       FIG. 9  shows a further embodiment comprising a further diverging nozzle  80  coupled to the burner assembly  600 . Said further diverging nozzle  80  is preferably a Laval nozzle. The velocity of the combustion gases is increased even further, which reduces the reaction times and leads to the formation of even smaller and more homogeneous particles  50 . Also adiabatic reduction of the gas temperature may take place in the shock wave SW. The temperature reduction in the shock wave SW is advantageous with regard to the condensation of the particles  50  and stopping of the chemical reactions leading to the formation of ion clusters, for example. A separate combustion chamber (not shown in the figures ) may be used before the diverging nozzle  80 .  
      The flame  100  is an intense source of heat. Consequently, the nozzles  12 ,  22 ,  32  ( FIG. 2 ),  42  ( FIG. 2 ),  80  may be provided with cooling means to prevent damage of the materials and/or to ensure problem-free flow of reactants. The cooling may be implemented by means of heat transfer medium, for example gas or water. The cooling may also be based on radiative cooling.  
      Referring to  FIG. 10 , One or more of the nozzles  12 ,  22 ,  32 ,  42  may have elements  26  with angular orientation to induce swirling, i.e. rotating motion to the gases. Examples of such swirl-inducing elements are vanes or flanges with tilted slots or tilted holes to modify the direction of gas flow. The nozzles may also comprise perforated or mesh-type elements to enhance turbulence.  
      The pressure p o  outside device  1000  may be altered by using an enclosure and a gas pump to affect the gas velocities, the particle collection efficiency, heat transfer rates and/or chemical reaction equilibria. Gas cleaning systems may be coupled to the process for example to remove chlorine-containing substances from exhaust gases.  
      Temperatures, flow rates, pressures, positions of the nozzles and position of the substrate  200  ( FIG. 2 ) are controlled by devices and components known by the person skilled in the art. The temperatures of the substrate  200  and the gases may be monitored by thermocouples and sensors based on emitted or absorbed spectral radiation. The proper form and symmetry of the flame  100  ( FIG. 2 ) may be monitored by an optical imaging system. Image sequences taken with short exposure times may assist in the monitoring of the degree of turbulence of the flame  100  and in the monitoring of the atomization process. Spectroscopical and fluorescent properties of the substrate  200  or of the produced material may be monitored on-line to assist in the control of the production of the particles  50 .  
      The atomizing gas  20  and/or reactants may also be supplied by a thermal plasma device, for example by using a direct-current non-transferred plasma torch, which is capable of accelerating the gas to a very high velocity and/or to a high temperature. Such plasma torches are known, for example, in the field of plasma spraying.  
      For the person skilled in the art, it will be clear that modifications and variations of the device, method and light-amplifying waveguide according to the present invention are perceivable. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.