MXPA01006354A - X-ray fluorescent emission analysis to determine material concentration - Google Patents

Abstract

A method and apparatus is disclosed to determine a concentration of dopant in soot that constitutes at least a portion of a soot preform (12) used to form an optical wave guide. The photon source (30) irradiates the soot preform on a mandrel (14). X-ray fluorescent emissions, from the irradiated soot are detected, and the concentration of dopant is determined based on the detected X-ray fluorescent emissions. Additionally, the concentration of dopant in layers of soot on the preform can be controlled by utilizing the detected X-ray fluorescent emissions to determine a deviation between a concentration of dopant in the soot and a predetermined concentration, and adjusting deposition conditions based on the deviation.

Description

FLUORESCENT X-RAY EMISSION ANALYSIS TO DETERMINE THE CONCENTRATION OF MATERIAL FIELD OF THE INVENTION The present invention relates to methods and an apparatus that use fluorescent emissions to determine a concentration of material in an object. More specifically, the present invention relates to methods and apparatus that use fluorescent emissions to determine a concentration of dopant in a soot preform used to form optical waveguide fibers ("optical fibers").

DESCRIPTION OF THE RELATED TECHNIQUE Almost always, an optical fiber includes a coating made of pure silica (SiO2) and a core made of silica doped with germania (GeO2). The germania dopant alters the refractive index of the silica in the nucleus. Core portions often contain different concentrations of germania, which results in different refractive indices along the diameter of the core. The distribution of refractive indices along the diameter of the core (ie, the profile of the refractive index) determines the operating characteristics of the optical fiber.

The optical fiber can be formed by a conventional process known as exterior vapor deposition ("OVD"). In general, the OVD process encompasses the formation of a soot preform by burning a gaseous mixture to produce soot-containing silica and germania, successively soot layers are deposited on a mandrel to form a core portion of the soot preform, burns a gaseous mixture to produce soot containing only silica, and successively soot layers are deposited in the core portion to form a coating portion of the soot preform. The soot preform is consolidated by the concretion to form a glass mold. An optical fiber is extracted from the glass mold. The concentrations of germania in the soot layers that form the core portion, mainly, determine the concentrations of germania along the diameter of the core of the resulting optical fiber. If the concentrations of germania in the soot layers can be measured, a soot preform can be evaluated to determine if the production of an optical fiber with a desired refractive index profile can be expected. Also, if the concentration of dopant can be determined in line, that is, during soot deposition, the concentration of dopant can be closely monitored and altered to obtain a desired profile. The Japanese patent application No. 59-106803 (Hara) and the patent of E.U.A. No. 4,618,975 (Glantschnig) describe techniques that use X-ray attenuation to evaluate in a non-destructive way the concentrations of germania in soot preforms. Both approaches measure the attenuation of X-rays at two energies. The scheme of Hara is that the ratio of attenuation of the dopant (Ge) to the matrix (Si) changes with the energy of X-ray photons. However, the Hara scheme is not particularly sensitive for soot preforms, because The ratio is almost constant over any practical X-ray energy scale. The Glantschnig method is based on the fact that the ratio of the attenuation of the dopant (absorption) to the attenuation of density (dispersion) changes with the energy of X-ray photons. Like the relation of Hara, the relationship of Glantschnig is almost constant on a practical energy scale for soot preforms. In this way, the Glantschnig method confuses density changes with changes in dopant concentration. These methods of attenuation of x-rays have additional disadvantages. For example, if a soot preform has multiple dopants, the X-ray attenuation due to a dopant can not be distinguished from the X-ray attenuation due to another dopant. On the other hand, the attenuation measurement requires the precise location of the preform within the measuring apparatus, and therefore, its realization is expensive. The patent of E.U.A. No. 4,292,341 (Marcuse) discloses methods for making in-line measurements of dopant concentration in a modified chemical vapor deposition process that does not form a soot preform, but instead immediately consolidates the soot in a glass mold. One of the methods described uses X-ray attenuation, which has many of the problems mentioned above. Another method measures the concentration of dopant by irradiating the glass mold with ultraviolet light and measuring the fluorescent emissions of the glass mold. It is believed that the last method will not work with a soot preform, since the soot is opaque to ultraviolet light and visible light.

BRIEF DESCRIPTION OF THE INVENTION The objects and advantages of the invention may be apparent from the following description. Other advantages of the invention can also be learned by practicing this invention. One aspect of the present invention includes a method for determining a concentration of soot dopant constituting at least a portion of a soot preform used to form an optical waveguide. The method comprises the steps of irradiating the soot with photons, detecting fluorescent X-ray emissions from the irradiated soot, and determining the concentration of dopant based on fluorescent X-ray emissions detected. Another aspect of the invention includes a method for controlling the making of a soot preform used to form an optical waveguide. The method comprises the steps to deposit the soot in the soot preform, irradiate the soot with photons, detect fluorescent X-ray emissions from the irradiated soot, use the fluorescent X-ray emissions detected to determine a deviation between a concentration of dopant in the soot and a predetermined concentration, and depositing the additional soot in the soot preform under set deposition conditions based on the deviation. Even another aspect of this invention includes an apparatus for determining the soot dopant concentration that constitutes at least a portion of a soot preform used to form an optical waveguide. The apparatus comprises a source of photons that irradiate the soot with photons, a fluorescent sensor that detects fluorescent X-ray emissions from the irradiated soot, and a determination device that defines the concentration of dopant in the soot based on the fluorescent emissions of the soot. X-rays detected. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings illustrate embodiments of the invention and together with the description serve to explain the principles thereof.

Figure 1 is a side view of an embodiment of an apparatus in accordance with the present invention for determining a concentration of dopant in a soot preform. Figure 2 is a cross-sectional view taken along line 2-2 of Figure 1 showing a source of photons and a fluorescent sensor. Figure 3 is a cross-sectional view taken along line 3-3 of Figure 1, illustrating a device for measuring thickness parameters. Figure 4 is a graph showing an example of soot weights determined from measurements made through a weight measuring device. Figure 5 is a graph illustrating an example of soot thicknesses determined from measurements made through the thickness parameter measuring device. 0 Figure 6 is a graph showing an example of soot densities determined from the weight and thickness of the soot. Figure 7 is a graph showing an example of fluorescence intensity measurements of the dopant. Figure 8 is a graph showing an example of fluorescence intensity measurements of the mandrel.

Figure 9 is a graph showing an example of the predicted dopant concentration.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Reference will now be made in detail to the preferred embodiments herein. Wherever possible, the same reference numbers will be used in all the drawings to designate the same or similar parts. The present invention determines the concentration of a dopant in a soot preform, preferably online, real time, measurement of layer by layer of concentration of dopant. More specifically, the present invention irradiates a soot preform with photons of sufficient energy to excite or ionize the atoms of the dopant. When the doping atoms become sufficiently ionized, they return to their ground state by a declination procedure known as fluorescence. In this declination procedure, the dopant atoms emit photons of energy, that is, fluorescent emissions. In general, the intensity of the fluorescent emissions will be proportional to the number of atoms of the dopant in the soot preform, as well as to the amount of soot deposited. Accordingly, the present invention detects the fluorescent emissions and determines the concentration of dopant based on the fluorescent emissions detected. Figures 1 to 3 illustrate a preferred embodiment of an apparatus 10 in accordance with the present invention for determining the concentration of dopant in a soot preform 12 of a mandrel 14. The apparatus 10 includes a photon counting 30, a fluorescence sensor 40 and a determination device 50. The apparatus may also include a thickness parameter measuring device 60 and a weight measuring device 20. The source of photons 30 irradiates the soot preform 12 with photons of sufficient energy to ionize the dopant atoms (eg, gemania). Effective photons include, for example, X-rays, soft X-rays, gamma rays and extreme ultraviolet light. The photon source 30, as shown in FIG. 2, preferably includes a 200-watt rhodium X-ray tube 32 (TruFocus Corp. TFS 6066 FGA / Rh) that excites a secondary selenium target 34 (used for binary soot) of GeO2 / SiO2). Different dopants can be detected using different secondary targets. The fluorescence sensor 40 detects the intensity of the fluorescent emissions of the dopant (preferably X-ray emissions) and provides a signal corresponding to the determination device 50. Preferably, the fluorescence sensor 40 includes a detector 42 placed therein. side of the soot preform 12 than the source of photons 30, as illustrated in Figure 2. The detector 42 may be, for example, a sealed proportional meter filled with xenon with a single channel analyzer circuit. The source of photons 20 and the fluorescence sensor 30, preferably form a relatively simple X-ray spectrometer, which includes associated protection 45. If it is desired to determine the concentrations of multiple dopants in the soot or detect the fluorescence from the mandrel 14, a more sophisticated spectrometer design will be necessary. The necessary design can be determined easily by one skilled in the art. For example, if it is desired to measure the fluorescence from the mandrel 14 through the binary soot Ge02 / Si02, the emissions of the rhodium X-ray tube 32 can be dispersed by epoxide to achieve excitation, and a multi-channel analyzer can be used with the sealed proportional counter filled with xenon 42 to resolve the fluorescence of the mandrel from the fluorescence of the dopant. The thickness parameter measuring device 60 measures a thickness parameter, such as radius or diameter, of the soot preform 12. Although the thickness parameter measuring device 60 appears misaligned to the right in Figure 1, for ease of illustration, it preferably measures along the central portion of the soot preform 12. As seen in Figure 3, the thickness parameter measuring device 60 can be a laser dimming micrometer that includes a source 62 that emits optical rays and a detector 64 that detects optical rays emitted by the source 62. Based on the detected optical rays, the detector 64 provides a signal to the determination device 50. This information allows a thickness parameter of the soot preform 12 to be determined for each transverse line, during the placement of soot layers. Devices available in the market that perform this function include Anritsu KL-154A and Keyence LS-5001. The weight measuring device 20 measures the weight of the soot preform 12 and provides a signal corresponding to the determining device 50. A preferred weight measuring device 20 includes a resistance load cell connected to one end of the mandrel 14 on which the soot preform 12 is formed. The other end of the mandrel 14 can be fixed to a pulse motor (not shown). The weight of the preform is recorded continuously during the placement of layers of soot. Variations due to wear of the preform are eliminated by the average of readings of the individual charged cells that is acquired during a rotation of the preform or an integral number of rotations of the preform. Variations due to the transverse location of the preform are adjusted by synchronizing the acquisition of weight with the transverse location of the preform. In other words, although the weight measurements are preferably continuous, an identifier or flag is related to the starting point of relevant weight measurements for each transverse line, and the starting point is the same on each transverse line. The weight of the preform in a given segment is the average of these synchronized and averaged readings. The weight of a segment is the weight gain observed from the previous segment. Preferably, the weight measuring device 20 measures the weight of the soot preform 12 continuously during the formation of the soot preform 12. The weight of any number of soot capable can be determined. The determination device 50 receives the signals from the fluorescence sensor 40, and in certain embodiments from the thickness parameter measuring device 60 and possibly also from the weight measuring device 20, and determines the concentration of dopant in the preform of soot 12. The determination device 50 can be a computer. The manner in which the determination device 50 defines the concentration of dopant will be explained below in more detail with respect to the preferred photons, i.e., X-rays. For an infinitely thin layer of soot, the fluorescence intensity of the purifier is directly proportional to the incident flow, the dopant concentration and the mass of the soot: (1) where ID = fluorescence intensity of the IP dopant = primary excitation current A = a constant of proportion WD = a weight fraction (concentration) of dopant Wt = weight of the soot lD = lP A WD pt (2) where p = soot density (grams / cm3) t = soot thickness (cm) For many-layer soot, the fluorescence intensity of a layer is reduced when the primary excitation and fluorescence of dopant are attenuated by any superimposed layer. The fluorescence of the L-layer purifier in a N-layer specimen is ID, L = lP A Tp? N, L (WdPt) TD, N, L (3) where ID.L = fluorescence intensity of the dopant layer L TP, N, L = primary excitation fraction transmitted through superimposed layers of soot, ie N layers a (L + 1) (WDP t)? _ = concentration, density and thickness of layer TD, N, L = fraction of fluorescence of dopant transmitted through superimposed soot layers, ie N layers a (L + 1).

The fluorescence intensity of the total meter is the sum of intensity of each of the N layers, N (4) L = 1 The fraction of X-rays transmitted through a layer is given by Beer-Lambert's law: T = i / l0 = exp (-μpt) (5) where T = fraction of transmitted X rays μ = mass attenuation coefficient (cm2 / gram) p = density (gram / cm3) t = thickness (cm) The coefficient of Mass attenuation for a compound or mixture is the sum of the mass mass of attenuation coefficients of the soot components: (6) where WN = weight fraction of the component N μN = mass attenuation coefficient of component N For a soot "binary" of GeO2 / Si02, the mass attenuation coefficient for fluorescence radiation attenuation of Ge by soot can be determined as follows: μ = WGEO2μGE02 + Wsiu2μSI02 (7) where WGEO2 = fraction of weight of Ge02 μGE? 2 = mass attenuation coefficient of GeO2 S? o2 = frac Weighting of SiO2 μs? o2 = mass attenuation coefficient of S¡O2 Assuming stoichiometry for Ge02 WGE = (AtWtGE / (AtWtGE + (2 AtWto))) WGEo2 (8) W0 = (AtWt0 / (AtWtGE + (2 AtWto ))) WGE02 (9) where AtWtoE = germanium atomic weight AtWtsi = silicon atomic weight AtWto = oxygen atomic weight In this way, WGE = 0.695 WGEOZ (10) WQ = 0.305 WGEo2 (11) Similarly, assuming the stoichiometry for S¡O2 WS? = (AtWts, / (AtWtS? + (2 AtWto))) WS? 02 (12) W0 = (AtWto / (AtWts? + (2 AtWto))) WS? O2 (13) In this way, WS? = 0.467 WS? O2 (14) W0 = 0.533 WS? O2 (15) For "binary" soot of Ge02 / Si02) where (16) then, WGE = 0.695 WGEOZ (17) (18) W0 = 0.305 WGE02 + 0.533 WS | 02 (19) Each fraction of elementary weight can be rewritten as a function of the WGEC dopant concentration »: WGE = 0.695 WQEOZ: (20) Ws? = 0.467 (1.0 - WGE02) (21) W0 = 0.305 WGE02 + 0.533 (1.0 -WGE02) (22) Finally, the mass attenuation coefficient μ of the soot can be expressed as a function of the concentration of mpurifier: μ = ( (0.695 WGE02) μce) + ((0.467 (1.0 - WGE02)) μs?) + ((0.305 WGE02 + 0.533 (1.0 - WGE02)) μo) (23) The attenuation depends not only on the material composition, but also on the X-ray energy. To estimate the attenuation of the primary excitation X-rays, the appropriate coefficient values are selected. For the attenuation of dopant emissions, other values are appropriate. A preferred embodiment employs a secondary target of selenium metal that provides selenium K "monochromatic" X-rays at 11.2 - 11.7 KeV. The resulting fluorescent X-rays of Ge K are predominantly 9.90 KeV. The mass attenuation coefficients have been tabulated by energy of elements and photons. For example, S.M. Seltzer, Calculation of Photon Mass Energy-Transfer and Mass Energy-Absorption Coefficients, Radiation Research, Vol. 136, 147-170 (1993), gives the following values: In summary, the fraction of X-rays transmitted through any layer can be calculated 1) by highlighting the energy of X-ray photons; 2) highlighting the concentration of dopant; 3) selecting the values of elementary mass attenuation coefficients for the energy of photons of interest from the published tabulations; 4) calculating a coefficient of attenuation of the material with the use of equation 23; 5) highlighting the density and thickness of the soot layer; and 6) calculating the transmitted fraction T using equation 5. The fraction transmitted by many layers is the product of the fractions transmitted by any of the N layers: T = Ti * T2 * TN (24) Given the concentration of the dopant, the density and the thickness of each soot layer, the attenuation of the primary excitation X-rays can be calculated when the outer soot layers penetrate to reach the soot layer L (Tp,, L). Also, the attenuation of fluorescent dopant radiations can be calculated when the soot layers penetrate outward from the L layer (TD, N, L).

Three preferred embodiments have been developed to determine the concentration of in-line dopant during soot deposition. A first mode determines the concentration of dopant using fluorescent emissions detected from the soot and on-line measurements of the weight and thickness of the soot. A second mode determines the concentration of dopant using detected fluorescent emissions of soot and fluorescent emissions detected (preferably X-ray emissions) from mandrel 14. A third mode, which can be used with an infinitely thick soot, determines the concentration of dopant using only Fluorescent emissions detected from soot. In the first preferred embodiment, the X-ray fluorescence of the dopant, the weight and the thickness parameter of the soot preform 12 are measured by the fluorescence sensor 30, the weight measuring device 20 and the measuring device thickness parameters 60, respectively after depositing each layer of soot in the soot preform 12. The weight Wt of each individual layer can be determined as the difference between the weight before and after depositing that layer. Similarly, the thickness t of each soot layer can be determined from the difference between the thickness parameter measured before and after depositing that layer. In this way, with a transverse length of known preform, the density p of the soot can be calculated for each layer. As an example, the weight data (Figure 4) and the thickness data (Figure 5) can be used to obtain the density data (Figure 6).

Accordingly, after having deposited the first soot layer, the values for the fluorescence intensity of the dopant lp, soot density p, and thickness of the soot t for that first soot layer can be made available. A value for (lp A) can be established by calibration, as explained below. The determination device 50 can then determine the concentration of WF dopant in the first layer by the iterative solution equation (2). Specifically, starting with a WD dopant concentration approximation, a predicted fluorescence intensity of mpurifier is calculated using equation (2). After comparing the predicted intensity with the observed intensity ID, the WD doping concentration estimate is adjusted and used to calculate a new predicted intensity. When the predicted intensity and the observed intensity agree reasonably, the determining device 50 has determined the concentration of the WD magnifier in the first layer. After having deposited the second soot layer, the values for the X-ray fluorescence intensity of the scavenger l for the two-layer preform, together with the density and thickness of the second layer, can be made available. Assuming that the concentration of dopant obtained for the first layer is correct, equation 4 is used, and the choice of doping concentration of the second layer is perfected until the fluorescence intensity of the multiplier predicted for the two-layer preform coincides reasonably with the observed one. This scheme is repeated with each layer of soot. In the second preferred embodiment, the preform mandrel can be made to flourish, and the fluorescence of the mandrel can be observed when the soot layers are deposited. The X-ray fluorescence of the dopant and the fluorescence of the mandrel are measured by the fluorescence sensor 30 each time a layer of soot is deposited on the soot preform 12. It is preferable to use a zirconia mandrel (ZrO2). Upon being excited by the X-rays of rhodium K (20.1 KeV) of the source of photons 30, the mandrel 14 will emit X-rays from Zr K to 15.7 KeV. The intensity of the Zr mandrel decreases after the deposition of each layer of soot. The decrease in intensity is related to the amount and composition of overlapping layers of soot, that is, to the densities of layers, thickness of the layers and concentrations of the purifier. This method does not require the measurement of the weight of the preform or the diameter of the preform. The intensity of the mandrel observed through a soot preform of N layers is determined by. (25) where IM = mandrel fluorescence intensity lP = intensity of primary X-rays that excite the mandrel B = a proportion constant TP, N = fraction of primary X-rays transmitted through N soot layers Tp, N = fraction of the fluorescent X-rays of the mandrel transmitted through N soot layers Tp >; N and TF, N are going to be evaluated as in equation 24 above. Now there are two simultaneous equations, one for the dopant fluorescence 4 and one for the fluorescence of the mandrel 25. There are two unknown. The first unknown is the concentration of the dopant W, which appears directly in the fluorescence equation of the dopant and in the transmitted terms T of fraction of both equations. The second unknown is the product of density p times thickness t, that is, the mass per unit area in grams / cm2. (iP A) and (b B) are established by calibration (as explained below). According to the above, the two equations can be solved at the same time by iteration. Figures 7 and 8 illustrate the X-ray emission intensity of the dopant and the mandrel, respectively, which is measured during a soot deposition experiment by CVD, which consists of a sequence of 5 (five) segments. The process parameters, such as oxygen flow rate, reactive agent flow rate and fuel to air ratio were varied in three segments, but remained constant within each segment. The procedure parameters for the third and fifth segments were identical.

It can be seen from Figures 7 and 8 that many layers of soot contribute to the X-ray emission counts that are being collected. In this way, for example, as shown in Figure 7, the first experimental segment presented an exponential progression of tales of X-ray emissions, but while more layers were deposited using the first conditions of the experimental procedure, the counts / emission step X-rays were established in a linear relationship. Also, as illustrated in FIG. 8, the intensity of the X-ray emission of the mandrel decreased continuously, since more soot layers were deposited, but the amount of decrease depended on the process parameters used to deposit the soot. Since overlapping layers can have an impact on subsequent layers, and since the degree to which these overlapping layers have an effect changes when more layers are deposited, it would be thought that it would be difficult to obtain useful information regarding individual layers of soot. who are depositing. However, as shown in Figure 9, by using the techniques described in the present invention, the obtained X-ray emission data can be used to obtain very useful information. Figure 9 illustrates the concentration of dopant, as determined by the simultaneous solution of the fluorescence equations of the dopant and the mandrel. It should be noted that when calculating the weight percentage of the dopant, as described herein, the effect of the superimposed layers and the contribution of the mandrel has already been counted. That is, the concentration / step of the dopant is displayed in a more linear relationship as opposed to the intensity / step in Figures 7 and 8, which results in more realistic and useful information about each step of doping deposition. In this way, as can be seen clearly in Figure 9, the second experimental segment showed an increase in the concentration of dopant, but suffered the increase in the concentration variability of the dopant. Also, it should be noted that the third and fifth conditions of the procedure that were identical resulted in nearly identical dopant / step curves. The third preferred embodiment is applied to homogeneous "infinitely thick" soot preforms. The infinite thickness is defined as the thickness from which 99.9% of the fluorescence intensity emanates. The radiations of greater depths are absorbed completely. The infinite thickness for the soot with 10% by weight of GeO2 / 90% by weight of SiO2 with density of 0.5 g / cm3, for example, is less than 1.0 millimeters. This mode would be exact for those soot preforms, where the outermost thickness of 1.0 millimeters was homogeneous. In this third preferred embodiment, only the x-ray fluorescence of the dopant is measured by the fluorescence sensor 30. The fluorescence intensity is b = b C exp (-μp) WD exp (-μp) (26) where b = fluorescence intensity of the dopant b = primary excitation current C = a proportion constant μp = mass attenuation coefficient for primary excitation by soot μp = mass attenuation coefficient for dopant fluorescence by soot WD = weight fraction ( contraction) of the dopant The product (b C) is established by calibration, as explained below. The mass attenuation coefficients are calculated by means of equation 23. This equation can be solved iteratively for the concentration of dopant WD, which appears directly and in μp and μF. To perform the above equations, it is not necessary to know neither the intensity of sources lP nor the constants of proportion A, B, C. Instead the products are necessary (lP A), (b B), (b C). The best values are determined by calibration. For example, a set of homogenous and thick soot preforms can be prepared on a scale of dopant concentrations. These preforms can be measured in accordance with the three preferred embodiments described above. Samples can be taken from the preforms and analyzed independently for the concentration of dopant by inductive coupled plasma emission spectrometry. Then for each preferred modality, the values are searched for (b A), (lP B), or (lP C) that produce the best match of known and predicted dopant concentrations. This can be done iteratively, by refining the choice of values to achieve concordance. Each modality has distinctive values. The present invention allows the measurement of the in-line dopant concentration. Among other things, online measurement allows the comparison of the measured dopant concentration with a predetermined concentration, so that the soot deposition conditions can be adjusted online based on the deviations between the measured and predetermined concentrations. For example, since soot is often deposited by burning a gaseous mixture containing silica and a scavenger, such as GeO2, the ratio of silica to scavenger in the gas mixture can be adjusted based on the deviation to obtain an index profile of desired replacement. It will be apparent to those skilled in the art that various modifications and variations may be made to the described embodiments of the present invention without departing from the spirit and scope thereof. For example, sources of monochromatic X-ray excitation, in particular secondary targets, have been considered. The polychromatic sources (tubes) are also appropriate. The equations described above would then be modified to integrate crossover (sum) energy. This requires the characterization of the source output on the energy and the evaluation of the mass attenuation coefficients on the energy.

As another example, in the design of the X-ray spectrometer, the soot area that illuminates is small. That is, the fluorescent intensities are not affected by the size of the preform. The equations given in the present are appropriate. In an alternative X-ray spectrometer design, the cross section of the entire preform can be seen. In this way, the fluorescence intensity of the dopant increases with the size of the preform. In fact, the intensity can be precisely balanced with the diameter of the preform. To adjust this, an intensity to diameter ratio depending on the diameter of the preform can be used. For this variation, the above equations must be modified to use the balanced intensity of the diameter instead of the gross intensity. Other embodiments of the invention will become apparent to those skilled in the art from consideration of the specification and practice of the invention described herein. The specification and the examples are intended to be considered as examples only, with a scope and true spirit of the invention which is indicated by the following claims.

Claims (11)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for determining a concentration of soot dopant constituting at least a portion of a soot preform used to form a waveguide, the method comprising the steps of: irradiating the soot with photons; detect the fluorescent X-ray emissions of the irradiated soot; and determining the concentration of dopant based on fluorescent X-ray emissions detected.

2. The method according to claim 1, further characterized in that the meter includes germania, the step of irradiating includes irradiation with photons that makes the germania flourish, and the step to detect includes the detection of fluorescent X-ray emissions of germania

3. The method according to claim 1, further characterized in that it comprises the steps for depositing the additional soot; irradiate at least the additional soot with photons; detect fluorescent X-ray emissions from at least the additional irradiated soot; and determining a purifier concentration in at least the additional soot based on fluorescent X-ray detected emissions.

4. - The method according to claim 1, further characterized in that it comprises the step to determine a parameter of thickness and weight of the soot, wherein the concentration of dopant is determined based on the fluorescent emissions of X-rays detected and the weight and parameter of certain thickness.

5. The method according to claim 1, further characterized in that it comprises the step to determine a soot attenuation characteristic by measuring emission attenuation of a mandrel in which the soot is deposited where the concentration of dopant is determined based on fluorescent X-ray emissions detected and the attenuation characteristic determined.

6. A method for controlling the preparation of a soot preform used to form an optical waveguide, the method comprising the steps of: depositing the soot in the soot preform; irradiate the soot with photons; detect fluorescent X-ray emissions from irradiated soot; using the fluorescent X-ray emissions detected to determine a deviation between a concentration of m purifier in the soot and a predetermined concentration; and depositing additional soot in the soot preform under set deposition conditions based on the deviation.

7. The method according to claim 6, further characterized in that the soot is deposited by burning a gaseous mixture containing silica and a dopant, and the step to deposit additional soot includes the adjustment of a ratio of silica to dopant in the mixture. gaseous based on the deviation.

8. An apparatus for determining the concentration of dopant in the soot that constitutes at least a portion of a soot preform used to form an optical waveguide, the apparatus comprising: a source of photons that irradiate the soot with photons; a fluorescence sensor that detects fluorescent x-ray emissions of the irradiated soot; and a determination device that defines a concentration of dopant in the soot based on fluorescent X-ray emissions detected.

9. The apparatus according to claim 8, further characterized in that the source of photons emits photons that make the germania flower, and the fluorescence sensor detects fluorescent X-ray emissions of the germania.

10. The apparatus according to claim 8, further characterized in that it comprises a thickness parameter measuring device that measures a thickness parameter of the soot preform and a weight measuring device that measures the weight of the soot, in where the determination device defines the concentration of impurifier based on fluorescent X-ray emissions detected and the weight and thickness parameter measured.

11. The apparatus according to claim 8, further characterized in that the fluorescence sensor detects the fluorescent X-ray emissions from a mandrel on which the soot is deposited and the determination device determines a soot attenuation characteristic based on the fluorescent X-ray emissions detected from the mandrel , wherein the determination device defines the concentration of dopant based on the detected X-ray fluorescent emissions and the determined attenuation characteristic.