Method of fabricating preforms doped with rare earth metal for optical fibers

Abstract: The present invention discloses a method of fabricating rare earth-doped preforms for optical fibers. A silica soot is deposited as a layer with high porosity on an inner surface of a silica-based tube by a modified chemical vapor deposition (MCVD) process at a temperature high enough to produce the silica soot but low enough to avoid sintering of the soot into the silica-based tube. The silica-based tube is then immersed in a solution including a rare earth element and optionally a codopant element for impregnation. The excess solution is drained and the silica-based tube is dried in a stream of chlorine and inert gas at an elevated temperature. Then, the rare earth element and the optional codopant element are oxidized under an oxygen partial pressure at a temperature high enough to overcome kinetic limitations against oxidation. Finally, the soot layer is consolidated while flowing oxygen and optionally a mixture of chlorides of a second codopant element at a sintering temperature at which the second codopant element reacts with oxygen to form codopant oxide which is delivered around the rare earth element oxide deposited in the soot layer. In the method of the current invention, one or more rare earth elements are codoped with preferred codopants including but not limited to Ge, Al, P and/or B to enhance the performance of the rare earth ions. Other dopants may also be used in conjunction with the preferred dopants for modifying the refractive index of the core.

Description: Field Of The Invention The invention relates to rare earth doped optical fibers for use as fiber lasers, fiber amplifiers and super-fluorescent sources. More particularly, the invention relates to a method for fabrication of a preform in which its core is doped with rare earth elements along with certain preferred codopants. Background Of The Invention When an optical fiber in which its core is doped with rare earth ions is pumped with radiation of a certain wavelength, the rare earth ions absorb the pump energy and subsequently emit radiation with wavelengths different from that of the pump radiation. This property of the rare earth ion has been utilized in making fiber lasers and amplifiers. Laser transitions are achieved over a wide range of wavelengths by incorporating an appropriate choice of rare earth dopants into host glass composition. A variety of rare earth ions including Nd, Er, Yb, Ho, Pr, Eu, Ce, Dy, Sm, Tb, Gd, Pm and Tm have been shown to provide useful laser actions. Numerous methods have been developed in the prior art to incorporate rare earth ions into silica glass. These methods include solution-doping (See, e.g., Townsend et al., "Solution doping technique for fabrication of rare earth doped fibers," Electron . Lett., 23(7):329, 1987; Tanaka, "Solution doping of a silica," U.S. Patent 5,474,588; Tanaka, "Erbium-doped silica optical fiber preform," U.S. Patent 5,526,459; Cognolato, "Method of fabricating optical fibers by solution doping, U.S. Patent 5,047,076), sol-gel (See, e.g., DiGiovanni et al., "A new optical fiber fabrication technique using sol-gel dip coating," Proceedings of OFC , vol. 4, paper WA2, 1991), aerosol (See, e.g., Morse et al., "Aerosol techniques for fiber core doping," Proceedings of OFC, vol. 4, paper WA3, 1991) and vapor phase techniques (See, e.g., Poole et al., "Fabrication of low loss optical fibers containing rare earth ions," Electron. Lett ., 21(17):737, 1985; MacChesney et al., "Multiconstituent optical fiber," U.S. Patent 4,666,247; Tuminelli et al., "Fabrication of high concentration rare earth doped optical fibers using chelates," J. Lightwave Tech., 8(11):1680, 1990) EP-Al-0 372 550 discloses a method of fabricating optical fibers by a solution-doping technique. In accordance with this prior art technique, the materials for the perform core are deposited by introducing the reactants into the tube, heated at temperatures above 1500°C. by a burner, to obtain a silica layer. Subsequently, an inert and dehydrated gas is made to flow through the tube. Subsequently, a non-aqueous solution of dopant precursors is introduced. The solution is lead to impregnate the soot. The soot is washed with a mixture of O 2 and Cl 2 to obtain a dehydration of the soot and the formation of Al 2 O 3 and Er 2 O 3 . EP-Al-0 466 932 relates to a method for manufacturing a rare earth element doped silica glass. A porous glass preform is impregnated with a rare earth element and aluminium and dried to cause the salts of the rare earth element and aluminium to deposit on the pores of the preform. The salts are oxidized for stabilisation. Subsequently, the porous glass preform is sintered after a drying step to remove the pores of the preform. Optionally, a separate dehydration step can be performed where the porous glass preform is dehydrated in a chlorine gas and oxygen containing atmosphere. It is known that the incorporation of certain modifiers or codopants in the proximity of the rare earth ions in the core can alleviate some deleterious effects on the emission of radiation with a desired wavelength and enhance the highly efficient conversion of the pump radiation to radiation of desired wavelength. These codopants include but are not restricted to Al, P, Ge and B. Although the beneficial effects of the preferred codopants and their presence in a close proximity of the rare earth ion have been well known, no attempt has been made to deliver the codopants preferentially to the close proximity of the rare earth ions. In the conventional doping methods, the rare earth element and preferred codopants are uniformly incorporated into a preform and their relative locations with respect to each other are usually dependent on a natural diffusion process, which occurs in the condensed phase and is very slow. In addition, although it is necessary for a rare earth element and a preferred codopant to have certain minimum concentrations to manifest their effective performance, methods to control independently a final concentration of each element in a preform to maximize their effects have not been available. For example, in prior art, P (or B) doping would be achieved by adding POCl 3 (or BCl 3 ) to the stream of SiCl 4 used to form the soot layer. This puts severe limitations on the porosity and amount of soot that is generated and consequently affects the amount of rare earth and codopant that can be incorporated. When a P (or B) doped soot is deposited, the soot tends to sinter at low temperatures. Such sintering reduces the porosity of the soot and, hence, the soot's ability to absorb the solution with the rare earth and codopant compounds. The temperature of the soot deposition can be reduced to increase the porosity. However, it was found that to achieve the desired amount of P the deposition temperature has to be reduced to an extent to which soot generation ceases. Thus, the methods of prior art cannot provide the means for incorporating rare earth elements and codopants at desired high levels, respectively. Summary Of Invention The present invention provides a method to fabricate a rare earth-doped preform for an optical fiber with a rare earth doped core. According to the invention, a porous silica soot layer is deposited on an inner surface of a silica-based substrate tube at a deposition temperature sufficient to obtain a desired porosity for the soot layer. The substrate tube with the porous silica soot layer is immersed in an impregnation solution containing at least one rare earth element for a time sufficient to allow the soot layer to become saturated with the solution. The porous silica soot layer is then dried with a stream of chlorine and inert gas at a drying temperature. The rare earth element impregnated in the soot layer is oxidized at an oxidation temperature to form a rare earth element oxide after the drying step. Finally, the porous silica soot layer is sintered into a glass layer, by flowing a mixture of oxygen and a codopant precursor over the soot layer at a sintering temperature of from about 1500 to about 2000 degrees C. so that a first codopant element oxide is formed in close proximity to the oxidized rare earth element in the soot layer. The codopant presursor is selected to enhance emission of radiation at a desired wavelength when the optical fiber drawn from the preform is pumped with radiation at a different wavelength. The porous silica soot layer may be deposited by a modified chemical vapor deposition process at a deposition temperature from about 1300°C to about 1800°C with a preferable deposition temperature at about 1600°C. The rare earth element has an atomic number of 57 through 71 such as Yb, Nd, and Er. The rare earth element is used in the form of a water or alcohol soluble compound such as chloride or nitrate to prepare the impregnation solution. The rare earth chloride used in the preferred embodiments is YbCl 3 6H 2 O, NdCI 3 6H 2 O, or ErCI 3 6H 2 O. The codopant element is one selected from the group consisting of Al,P, Ge and B, preferably in the form of a water or alcohol soluble compound such as chloride in the case of aluminum. A typical duration of the impregnation time is at least 0.5 hour, preferably about 1 hour. In a preferred embodiment, the porous silica soot layer is dried from the impregnation solution first with a stream of inert gas and subsequently with said stream of chlorine and inert gas at said drying temperature. The temperature for the drying step ranges from 600°C to 1200°C, preferably 800-1000°C. The oxidation temperature ranges from 600°C to 1000°C. In one embodiment, a second codopant element is contained in the impregnation solution so that a second codopant element oxide is formed in the silica soot layer when the rare earth element is oxidized after the drying step. The second codopant element is also selected to enhance emission of radiation at a desired wavelength when the optical fiber is pumped with radiation at a different wavelength. Brief Description Of The Drawing Figure 1 is a schematic illustration of a deposition of a porous silica soot layer according to the invention. Figure 2 is a schematic illustration of a solution impregnation of the porous silica soot layer according to the invention. Figure 3 is a schematic illustration of an oxidation of a rare earth element and a codopant element in the silica soot layer according to the invention. Figure 4 is a schematic illustration of a sintering process of the silica soot layer according to the invention. Detailed Description Of The Invention The present invention discloses a method for delivering the desired codopants to a close proximity of rare earth ions so that highly efficient energy conversion can be achieved. The invention also allows an independent control of the concentration of rare earth element and those of preferred codopants, and facilitates the incorporation of high levels of dopants to further enhance the efficiency of energy conversion. These advantages are achieved by uncoupling the silica soot deposition step from the dopant incorporation steps so that the deposition of pure silica produces an excellent porosity, which, in turn, allows the incorporation of high amounts of dopants and codopants. The present invention discloses a method of fabricating rare earth-doped preforms for optical fibers. A silica soot is deposited as a layer with high porosity on an inner surface of a silica based tube by a modified chemical vapor deposition (MCVD) process at a temperature high enough to produce the silica soot but low enough to avoid sintering of the soot into the silica-based tube. The silica-based tube is then immersed in a solution including a rare earth element and preferably a codopant element for impregnation. The excess solution is drained and the silica-based tube is dried first in a stream of inert gas and subsequently-in a stream of chlorine and inert gas at an elevated temperature. Then, the rare earth element and the codopant element are oxidized under an oxygen partial pressure at a temperature high enough to overcome kinetic limitations against oxidation. Finally, the soot layer is consolidated while flowing a mixture of chlorides of a codopant compound and oxygen at a sintering temperature at which the codopant compound reacts with oxygen to form codopant oxide which is delivered around the rare earth element deposited in the soot layer. In the method of the current invention, one or more rare earth elements are codoped with preferred codopants including but not limited to Ge, Al, P and/or B to enhance the performance of the rare earth ions. Other dopants may also be used in conjunction with the preferred dopants for modifying the refractive index of the core. The present invention is now described in detail with reference to the accompanying figures. At the first step, a silica soot layer is deposited on the inner surface of a silica-based substrate tube by an MCVD process as shown in Figure 1 under conditions where the silica soot layer does not sinter into a layer of glass. Rather, the silica soot layer is deposited in such a way that it has substantial porosity (i.e., low density) to enable the subsequent doping solution to penetrate into the silica soot layer. There is a window of acceptable soot density. If the soot density is too low, the silica soot layer does not have adequate strength to withstand a subsequent solution impregnation step and tends to flake. On the other hand, if the density is too high, the silica soot layer is not able to let the impregnation solution penetrate into the entire layer which would result in superficially uniform doping. Thus, the soot density is one of the important parameters for controlling the dopant concentration in the final product. One of the processing parameters that can be used to vary the soot density is the deposition temperature. The higher the soot deposition temperature, the greater the degree of sintering and the higher the soot density. In general, the deposition temperature in the invention ranges from about 1300 °C and about 1800°C, and preferably at 1600 °C. The silica-based substrate tube with the porous silica soot layer is then immersed into a premixed solution comprising a rare earth compound(s) as shown in Figure 2. To prepare the rare earth-containing solution, one or more rare earth compounds, including but not limited to chlorides and nitrates, are dissolved into an aqueous or alcoholic solution. Among the rare earth elements of atomic numbers 57 through 71, Yb, Nd and Er are the most common rare earth elements used in the present invention. For example, chlorides of these elements (or other soluble compounds of these elements), YbCl 3 6H 2 O, NdCl 3 6H 2 O, and ErCl 3 6H 2 O, alone or in combination, are dissolved in ethanol (or other alcohol or water) at the concentrations between about 0.1 and about 30 g/L. The preferred codopants that are to be delivered in close proximity to the rare earth element include but are not limited to Al, P, Ge and B. The addition of these codopants is essential to promote the solubility of the rare earth element which typically has limited miscibility in silica glass and/or to modify the local phonon energy of the glass. One or more codopants are incorporated into the silica soot layer during the sintering step in which the porous soot layer with the soot particles coated with rare earth and codopant oxides is sintered into a glass layer to form an optical fiber preform. This sintering step (also referred to as soot consolidation) is carried out while flowing a mixture of gaseous codopant precursors and oxygen as shown in Figure 4. The gaseous codopant precursors can be in the form of chlorides. For example, POCl 3 and/or GeCl 4 and/or BCl 3 are used in a preferred example. The carrier gas flows through the POCl 3 bubbler and is maintained between about 5 and about 300 sccm. In another example, aluminum codopant may also be achieved by flowing AICI 3 along with oxygen during sintering rather than using an impregnation solution with an aluminum compound. The sintering temperature ranges from about 1500 °C to about 2000 °C and preferably at around 1800 °C. At the sintering temperature, POCl 3 (and/or GeCl 4 and/or BCl 3 ) reacts with oxygen to form P 2 O 5 (and/or GeO 2 and/or B 2 O 3 ). These oxides are delivered around the rare earth element before the porous layer is consolidated into a dense glass layer. Thus the preform fabricated in this manner has the preferred codopants delivered in close proximity of the rare earth element(s). This delivery method of the codopants also alleviates limitations on the amount of codopants that can be incorporated into the glass. The amount of codopants needed depends on the concentration of rare earth that is desired in the fiber. The higher the concentration, the shorter the length of the fiber that will provide the needed energy conversion, provided that the local environment around the rare earth is appropriate. To ensure that the rare earth has the right local environment, higher concentrations of the preferred codopants are needed as the rare earth concentration is increased. With regard to aluminum as a codopant, its concentration in the soot layer can be increased for example by dissolving additional amount of aluminum compound in the impregnation solution. However, beyond a certain concentration, aluminum starts to separate from the soot layer as crystallites (devitrification) and causes scattering. This can be alleviated if the preform is codoped with P. Accordingly, a substantial amount of P codoping is needed not only to improve the performance of the rare earth ions in the glass but also to ensure that the high level of Al dopant will not lead to devirification. After the consolidation step, the preform is collapsed as standard MCVD preform. The collapse temperatures are adjusted to minimize the non-circularity of the core and can be raised up to about 2300 °C. The high temperatures used for preform collapse pose a significant problem in terms of dopant volatilization via halogenation. This could lead to significant radial non-uniformity in dopant distribution. The dopant volatilization may be suppressed by using methods known to those skilled in the art. The present invention can be further understood through the following embodiments. Embodiment 1: Ytterbium (Yb)-doped preforms A silica soot layer is deposited at a temperature ranging from 1300 C to 1800 °C, preferably at 1600°C. An impregnation solution is prepared by dissolving YbCl 3 6H 2 O and AlCl 3 6H 2 O (or other soluble Yb and Al compounds) in ethanol (or other alcohol or water) ranging from 0.5 to 30 g/L, preferably at 10 g/L and 0.5 to 150 g/L, preferably at 40 g/L, respectively. The soot layer is impregnated with the solution and is subsequently dried preferably in a chlorine atmosphere. The YbCl 3 □6H 2 O and AlCl 3 □6H 2 O are oxidized on the soot layer. The soot layer is then sintered at a temperature ranging from 1500 °C to 2000 °C, preferably at 1800 °C in a POCl 3 environment. The carrier gas flow to the POCl 3 bubbler is maintained at 5 to 200 sccm, preferably at 60 sccm, to deliver POCl 3 . The fiber made from the preform has a numerical aperture ranging from 0.07 to 0.25 with a typical number of 0.1 with a core diameter ranging from 3 to 11 µm with a typical diameter of 8.0 µm. The preform is drawn so that the fiber can be core pumped or clad pumped with suitable pump radiation that is absorbed by the Yb ions. The pump wavelength of 915 nm is used in one example. The fiber thus made has exhibited near theoretical quantum efficiencies. Embodiment 2: Neodymium (Nd)-doped preforms A silica soot layer is deposited at a temperature ranging from 1300 °C to 800°C, preferably at 1600 C. An impregnation solution is prepared by dissolving NdCl 3 6H 2 O and AlCl 3 6H 2 O (or other soluble Nd and Al compounds) in ethanol (or other alcohol or water) ranging from 0.5 to 30 g/L, preferably at 7 g/L and 0.5 to 150 g/L, preferably at 27 g/L, respectively. The soot layer is impregnated with the solution and is subsequently dried preferably in a chlorine atmosphere. The NdCl 3 □6H 2 O and AlCl 3 □6H 2 O are oxidized. The soot layer is then sintered at a temperature ranging from 1600 °C to 2000 °C, preferably at 1800 °C in a POCl 3 environment. The carrier gas flow to the POCl 3 bubbler is maintained at 5 to 300 sccm, preferably at 45 sccm, to deliver POCl 3 . The fiber made from the preform has a numerical aperture ranging from 0.07 to 0.25 with a typical number of 0.1 with a core diameter ranging from 3 to 10 µm with a typical diameter of 6.5 µm. The preform is drawn so that the fiber can be core pumped or clad pumped with suitable pump radiation that is absorbed by the Nd ions. The pump wavelength of 834 nm is used in one example. The fiber thus made has exhibited near theoretical quantum efficiencies. Embodiment 3: Erbium (Er)-doped preforms A silica soot layer is deposited at a temperature ranging from 1300 °C to 1800 °C, preferably at 1600 °C. An impregnation solution is prepared by dissolving ErCl 3 6H 2 O and AlCl 3 6H 2 O (or other soluble Er and Al compounds) in ethanol (or other alcohol or water) ranging from 0.1 to 3 g/L, preferably at 0.5 g/L and 5 to 200 g/L, preferably at 100 g/L, respectively. The soot layer is impregnated with the solution and is subsequently dried preferably in a chlorine atmosphere. The ErCl 3 6H 2 O and AlCl 3 6H 2 O are oxidized. The soot layer is then sintered at a temperature ranging from 1500 °C to 2000 °C in a POCl 3 and/or GeCl 4 environment. The carrier gas flow to POCl 3 and/or GeCl 4 bubbler is maintained at 5 to 150 sccm, preferably at 40 sccm and 0 to 300 sccm, preferably at 60 sccm, to deliver POCl 3 and GeCl 4 , respectively. The fiber made from the preform has a numerical aperture ranging from 0.13 to 0.35 with a typical number of 0.2 with a core diameter ranging from 2 to 9 µm with a typical diameter of 3.4 µm. The preform is drawn so that the fiber can be core pumped with suitable pump radiation that is absorbed by the Er ions. The pump wavelength of 980 nm or 1480 nm is used in one example. The fiber thus made has exhibited near theoretical quantum efficiencies. Embodiment 4: Erbium/Ytterbium (Er/Yb) co-doped preforms The method of the invention may also be used to make preforms whose core is doped with more than one rare earth dopant. Such codoping of rare earth elements allows for novel pumping schemes and alleviates certain difficulties faced with directly pumping certain rare earth ions. In this method a first rare earth is co-doped with a second rare earth which can easily absorb a pump radiation. The co-doped fiber is pumped with radiation that is easily absorbed by the second rare earth. The second rare earth converts this energy into a radiation that is easily absorbed by the first rare earth, which in turn emits the desired radiation for laser action or signal amplification. With an appropriate ratio of the two rare earth ions and a suitable host glass composition, an efficient fiber laser/amplifier can be made. One such example is Er/Yb co-doped fiber. A silica soot layer is deposited at a temperature ranging from 1300 °C to 1800°C, preferably at 1600 C. An impregnation solution is prepared by dissolving YbCl 3 6H 2 O, ErCl 3 6H 2 O and AlCl 3 6H 2 O (or other soluble Yb, Er and Al compounds) in ethanol (or other alcohol or water) ranging from 0.5 to 30 g/L, preferably at 10 g/L and 0.05 to 3 g/L, preferably at 0.5 g/L, and 0.5 to 15 g/L, preferably 30 g/L, respectively. The YbCl 3 6H 2 O, ErCl 3 6H 2 O and AlCl 3 6H 2 O are oxidized. The soot layer is impregnated with the solution and is subsequently dried preferably in a chlorine atmosphere. The soot layer is then sintered at a temperature ranging from 1500°C to 2000°C, preferably at 1800°C in a POCl 3 environment. The carrier gas flow to the POCl 3 bubbler is maintained at 5 to 300 sccm, preferably at 60 sccm, to deliver POCl 3 . The fiber made from the preform has a numerical aperture ranging from 0.07 to 0.25 with a typical number of 0.12 with a core diameter ranging from 3 to 11 µm with a typical diameter of 6.0 µm. The preform is drawn so that the fiber can be core pumped or clad pumped with suitable pump radiation that is absorbed by the Yb ions. The pump wavelength of 1064 nm is used in one example. The fiber thus made has exhibited good quantum efficiencies. The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.

Claims: A method of fabricating a rare earth doped preform from which an optical fiber having a rare earth doped core is to be drawn, characterized by : depositing a silica soot layer on an inner surface of a silica-based substrate tube at a deposition temperature sufficient to obtain a desired porosity for the soot layer; immersing the substrate tube with the deposited porous silica soot layer in an impregnation solution containing a rare earth element for a time sufficient to allow the silica soot layer to become saturated with the solution; drying the saturated silica soot layer with a stream of chlorine and an inert gas at a drying temperature; oxidizing the rare earth element impregnated in the silica soot layer at an oxidation temperature after the drying step; and sintering the porous silica soot layer into a glass layer, including flowing a mixture of oxygen and at least one codopant precursor over the soot layer at a sintering temperature ranging from about 1500°C to about 2000° C, thus forming at least a first codopant element oxide in close proximity to the oxidized rare earth element in the silica soot layer, wherein the codopant precursor is selected to enhance emission of radiation at a desired wavelength when the optical fiber is pumped with radiation at a different wavelength. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 1, wherein said porous silica soot layer is deposited by a modified chemical vapor deposition process. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 1 or 2, wherein said deposition temperature ranges from about 1300°C to about 1800°C. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 3, wherein said deposition temperature is at about 1600°C. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said rare earth element has an atomic number of 57 through 71. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 6, wherein said rare earth element is the one selected from the group consisting of Yb, Nd and Er. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said rare earth element is in the form of a water or alcohol soluble compound. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 7, wherein said rare earth compound is a chloride. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 8, wherein said rare earth chloride is one selected from the group consisting of YbCl 3 6H 2 O, NdCl 3 6H 2 O and ErCl 3 6H 2 O. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 7, wherein said rare earth compound is a nitrate. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said codopant element is one selected from the group consisting of Al, P, Ge and B. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said codopant element is in the form of a water or alcohol soluble compound. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 12, wherein said codopant compound is AlCl 3 6H 2 O. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 12, wherein said codopant compound is Al(NO 3 ) 3 9H 2 O. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein the duration for the immersing step is at least 0.5 hour. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 15, wherein the duration for the immersing step is about 1 hour. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said drying temperature ranges from 600°C to 1200°C. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 8, wherein said drying temperature is from 800°C to 1000°C. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said drying step further comprises a step of preliminary drying said porous silica soot layer with a stream of inert gas. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 19, wherein the duration for said preliminary drying step ranged from about 0.5 hour to about 1 hour. The method of fabricating a rate earth-doped preform for an optical fiber according to claim 19, wherein said inert gas is nitrogen. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said oxidation temperature ranges from 600°C to 1000°C. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 1, wherein said codopant precursor is a chloride selected from the group consisting of POCl 3 , GeCl 4 and BCl 3 . The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said sintering step is conducted at a temperature between about 1500°C and about 2000°C. The method of fabricating a rare earth-doped preform for an optical fiber according to claim 24, wherein said sintering step is conducted at a temperature of about 1800°C. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, wherein said codopant is used to promote the solubility of said rare earth element. The method of fabricating a rare earth-doped preform for an optical giber according to any of the preceding claims, wherein said codopant is used to modify phonon energy of said silica soot layer. The method of fabricating a rare earth-doped preform for an optical fiber according to any of the preceding claims, including containing a codopant element in the impregnation solution so that a second codopant element oxide is formed in the silica soot layer during the oxidizing step, wherein the codopant element is selected to enhance emission of radiation at a desired wavelength when the optical fiber is pumped with radiation at a different wavelength.

Inventors: TANKALA KANISHKA, TANKALA, KANISHKA
Applicants: SPECTRAN CORP, SPECTRAN CORPORATION
Application year: 2000
Main classification: C03B37
IPC codes: C03B37, C03B27