CN1500069A - Preparation method of optical fiber doped with rare earth - Google Patents

Abstract

The present invention provides an improved method of preparing rare earth-doped preforms and fibers by combining MCVD techniques with solution doping methods, said method comprising: forming a matching or depressed cladding structure within a quartz glass substrate tube, followed by , deposit an unsintered particle layer containing _GeO2 and _P2O5 to form _the core, and, by soaking the porous soot layer in an alcohol containing a defined ratio of codopants such as _AlCl3 /Al( _NO3 ) ₃ RE salts solution doping in aqueous solution, by controlling the porosity of soot, soaking time, solution concentration and co-dopant ratio, the required RE ion concentration is obtained at the core and the core-cladding interface defects are minimized, after that, The porous deposition layer containing RE is dried, oxidized, dehydrated and sintered, and collapsed at high temperature to prepare a preform, which is coated with a quartz tube of appropriate size, and then made into a fiber by fiber drawing.

Description

Preparation method of optical fiber doped with rare earth

technical field

The invention relates to a preparation method of rare earth-doped optical fiber.

Background technique

Optical fibers doped with rare earths (REs) have great potential for applications in many fields including amplifiers, fiber lasers, and sensors. Rare earth oxides are doped as active species into the core of such fibers. Doping with various rare earths has been shown to produce lasing and amplification in several wavelength ranges, however, for communications, erbium-doped fiber (EDF) remains the most important because it operates at the same wavelength as the third lowest The lossy optical window is matched.

Erbium-doped fiber amplifiers (EDFAs), which operate in a low loss window of about 1.53 μm, play a key role in today's high-capacity communication systems. It is capable of amplifying optical signals directly, independent of the modulation format. The opto-repeaters used to date in such systems are 3R elements, but their amplification in discrete wavelength ranges is limited. EDFA has the ability to simultaneously amplify several optical channels in one fiber, can implement WDM technology (wavelength division multiplexing technology), and has the potential to increase the bandwidth of long-distance transmission systems from Gb/s to Tb/s. Therefore, in the working wavelength range, it exhibits high gain, large bandwidth, low noise, polarization insensitive gain, significantly reduced crosstalk interference and low lead-in loss. The success of future high-capacity optical networks and transmission systems largely depends on the development of high-efficiency EDFAs.

Can refer to the article (Electronics Letters, Vol.23 (1987) p329) "Solution-doping technique forfabrication of rare-earth doped optical fiber" of Townsend J.E., Poole S.B. and Payne D.N. The preform of the refractive index distribution and the required core-skin structure, at the same time, the solution doping is used to add active ions. The method comprises the steps of:

i. A conventional cladding layer doped with _P2O5 and F is deposited in a high silica glass substrate tube to form a matched cladding or depressed cladding _type structure.

ii. At a lower temperature, a core layer with a predetermined composition comprising an index increasing dopant such as _GeO2 is deposited to form an unsintered porous soot.

iii. Immerse the tube with the deposited layer in an aqueous solution of the dopant precursor (typical concentration 0.1 M) for up to 1 hour. Although rare earth halides are most used, any soluble form of the dopant ion is suitable for preparing the solution.

iv. After soaking, the tube was rinsed with acetone and remounted on the lathe.

v. Dehydrating and sintering the RE-containing core layer to obtain a transparent glass layer. Dehydration was performed with chlorine at 600°C. Using a Cl ₂ /O ₂ ratio of 5:2 can reduce the ^OH- level to less than 1 ppm when the drying time exceeds 30 minutes.

vi. Collapse in the usual way to obtain a solid glass rod called a preform.

vii. Perform conventional fiber drawing.

Reference may also be made to DiGiovanni D.J.'s paper (SPIE Vol. 1373(1990) p2) "Fabrication of rare-earth-doped optical fiber", in which a substrate tube with a porous core is soaked in nitric acid containing the RE ions required for the presence of In aqueous or alcoholic solutions of salt or chloride. The tubes were dewatered, dried and remounted on the lathe. Dehydration was performed by passing dry chlorine gas through the tube at about 900°C for 1 hour. After dehydration, the layer is sintered and the tube is collapsed and drawn into fibers.

You can refer to another article by Ainslie BJ, Craig SP, Davey ST and Wakefield B. (Material Letters, Vol.6(1988) p-139) "The fabrication, assessment and optical properties of high-concentration Nd ³⁺ and Er ^{3 +} doped silicabased fibers", in which optical fibers based on Al ₂ O ₃ -P ₂ O ₅ -SiO ₂ host glass doped with high concentrations of Nd ³⁺ and Er ³⁺ were fabricated and quantified by the solution method . After the cladding was deposited, _P2O5 -doped quartz soot was deposited at _a lower temperature. The prepared tubes were soaked in an alcoholic solution containing 1 M Al(NO ₃ ) ₃ + various concentrations of ErCl ₃ and NdCl ₃ for 1 hour. The tube is then blown dry and collapsed into a preform in the usual manner. Al is said to be the key component to obtain high RE concentrations in the core center without clustering effects. It was further found that the Al and RE distributions were combined in some way, thereby preventing the RE ions from volatilizing. A dip in the content of P and _GeO2 at the center of the core has been observed.

Reference can also be made to the U.S. Patent (Patent No.: 5,005,175 (1991)) "Erbium doped fiber amplifier" of Desuvire et al., wherein the fiber used for the optical amplifier comprises a single-mode core-doped fiber with erbium, and has the following RE ion distribution: The RE ions have a radius of less than 1.9 μm, while the pump signal mode has a radius of more than 3 μm. The numerical aperture (NA) of the fiber is 0.2-0.35, and, for improved efficiency, the core is doped with both Al and Ge oxides. When the Er-doped core radius is equal to or lower than that of the fiber's pump mode, it is believed that each Er atom at the core cross-section is exposed to substantially the same level of pump mode high intensity fraction. It is reported that fibers with this design have higher gain and lower threshold than conventional Er-doped fiber amplifiers. The radius of the conventional Er-doped core is larger than that of the pump mode, so the erbium atoms at the edge of the core cannot get enough pump photon flux to produce a net gain.

According to G.S.Roba's U.S. Patent (Patent No.: 5,491,581 (1996)) "Rare earthdoped optical fiber amplifiers", in which, it is reported that the high concentration of germanium oxide at the core for increasing the NA value of the fiber can be at the core-cladding interface Residual stress is generated at the place, because the viscosity and expansion coefficient of the two are different. It is believed that the residual stress in turn causes an undesired increase in the background loss of the fiber.

According to U.S. Patent (Patent No.: 5,778,129 (1998)) of Shukunami et al. "Dopedoptical fiber having core and clad structure are used for increasing theamplification band of an optical amplifier using the optical fiber", wherein, after the cladding is formed inside the quartz tube, The porous core layer is deposited by the MCVD method, and Er as an active ion is infiltrated into the porous core by the solution doping method, and then vitrified and slumped to form a preform. The solution also contains a compound of Al, ie chloride, in order to co-dope the core with Al and thus enlarge the amplification band. The glass doped with Er and Al constitutes the first region of the core. Surrounding this area are the second and third areas of the core. The third region contains Ge in order to increase the refractory index. The impurity concentration of the second region is lower than that of the first and third regions, and therefore, its RI value is also lower. The second region acts as a barrier against the diffusion of active dopants.

Reference can also be made to Tanaka, D. et al.'s U.S. Patent (Patent No.: 5474588 (1995)) "Solution doping of a silica with erbium, aluminum and phosphorus to form an optical fiber", which describes the manufacture of Er-doped silicon oxide method, that is: depositing a layer of quartz glass soot (VAD device) on a seed rod to form a porous soot preform, which is soaked in a compound containing an erbium compound, an aluminum compound and a an alcoholic solution of a phosphate ester, and drying the preform to form a soot preform containing Er, Al and P. The drying is carried out at 60-70° C. for 24-240 hours in an atmosphere of nitrogen or inert gas. The dried soot preform is heated and dehydrated at 950-1050°C for 2.5-3.5 hours in a helium atmosphere containing 0.25-0.35% chlorine, and further heated at 1400-1600°C for 3-5 hours, to make it transparent. An erbium-doped glass preform is thus formed. Due to the presence of phosphorus, segregation of _AlCl3 is suppressed during preform formation, and as a result, a doping concentration of Al ions with a high level (>3 wt%) can be achieved. It is reported that the doping concentrations and component ratios of Er, Al, and P ions are extremely precise and uniform in the radial and longitudinal directions.

However, the aforementioned methods have some disadvantages, namely:

1. What is obtained at the core is a step-like RE distribution, resulting in poor overlap between the pump signal and RE ions, which reduces the pumping efficiency.

2. The step-like RE distribution requires a high numerical aperture (NA) at the core or limits the RE in the central region (ie: 50% of the total core area) to improve pumping efficiency, which in turn leads to the following disadvantages:

i) Doping RE only in selected parts of the core is extremely difficult and its reproducibility suffers due to the sensitivity of the process to process parameters in the various process stages such as deposition, solution doping, drying and sintering.

ii) Increasing the NA value of the fiber while reducing the core area requires a high concentration of germanium oxide in a smaller core region, consequently increasing the likelihood of impregnations forming in the center due to volatilization during sintering and slumping.

iii) For preforms with high NA values (>0.20), the high germanium oxide concentration at the core reduces the viscosity of the glass, making the process very sensitive especially to the temperature during the porous soot layer deposition and sintering steps.

iv) Increased temperature sensitivity during porous soot deposition leads to variations in composition and soot density along the length of the tube.

v) High germanium oxide concentration at the core can generate residual stress at the core-cladding interface due to the difference in viscosity and expansion coefficient. Residual stress can undesirably increase the background loss of the fiber.

vi) It is believed that: Residual stress produces polarization mode dispersion (PMD), causing severe impairment of capacity, including pulse broadening. Since the magnitude of the PMD is not stable at a given wavelength, passive compensation becomes impossible.

3. The dehydration and sintering of the soot layer containing RE chlorides is critical because the process changes the composition by evaporation and also diffuses dopant salts as well as _GeO2 present at the core.

purpose of invention

The main object of the present invention is to provide a method for preparing a rare earth-doped optical fiber which overcomes the aforementioned disadvantages.

Another object of the present invention is to provide fibers possessing a controllable RE distribution, in particular the distribution of erbium in the doped region is similar to the distribution of the pump beam intensity in the fiber, with a maximum concentration at the center, and as a result, can significantly improve overlap between the two.

Yet another object of the present invention is to provide an optical fiber in which the pump beam distribution radius is equal to or greater than that of the RE ion distribution at the core, so as to increase the exposure of all active ions to the pump light, resulting in improved pumping in the fiber. Pu conversion efficiency.

Yet another object of the present invention is to provide a method of controlling the distribution of Gaussian RE in the radial direction in the core.

Yet another object of the present invention is to achieve high optical gain in the fiber with an NA value only close to 0.20, thereby avoiding a wide range of compositional variations between the core and the cladding glass, eliminating problems such as residual stress and PMD.

Yet another object of the present invention is to develop erbium-doped fibers suitable for amplifying input signals, which fiber NA and mode field diameters are not significantly different from signal-transmitting fibers, thereby facilitating splicing.

Yet another object of the present invention is to reduce the possibility of changes in the composition of the particulate core due to evaporation of the RE salt during drying and sintering.

Yet another object of the present invention is to reduce the amount of germanium halide required to obtain the desired NA value in the fiber.

Yet another object of the present invention is to provide a method in which the numerical aperture of the fiber is 0.10-0.30, the RE concentration at the core is maintained between 50-6000 ppm, and together with the change in the RE profile in the doped region, Fabricated fibers suitable for use as amplifiers, fiber lasers and sensors for different applications.

Summary of the invention

The novelty of the present invention lies in the control of the concentration profile of RE ions in the collapsed preform by minimizing the evaporation of RE salts and preventing the diffusion of rare earth ions due to subsequent heat treatment. To achieve this goal, the optimum soot density after deposition is estimated to be 0.3-0.5. The inventive step of the present invention consists in converting the RE salts into oxides by gradually heating the tube to higher temperatures while maintaining an oxidizing atmosphere inside, thereby minimizing the occurrence of RE during subsequent processing. Possibility of evaporation, since oxides have very high melting points compared to halides/nitrates. This step also helps to remove the solvent trapped in the porous layer. The inventive step also includes gradually raising the temperature of the porous layer containing RE to the sintering temperature and higher temperature with a gradient of 50-200° C., so as to perform sintering and further fix RE ions at required positions. The gradient depends on the composition of the host glass and the concentration of Er/Al in the core layer. As a result, the efficiency of RE from solution into the core layer is significantly improved, making the process more efficient and economical. The RE distribution along the transverse direction at the core depends on the density of the porous soot layer, soaking time, and processing conditions during oxidation, sintering, and slumping.

Sintering the porous core in a _GeO2 rich atmosphere with the addition of oxygen and helium is another inventive step of the present invention which reduces the amount of _GeCl4 required to obtain the required NA value and makes the process more economical . During the sintering step, pure _GeCl4 is supplied with input oxygen at a temperature of 200-1400°C, the amount depends on the required NA value in the fiber. Sintering is continued by gradually increasing the temperature until a transparent glass layer is formed.

Detailed description of the invention

Accordingly, the present invention provides an improved method of making a rare earth doped optical fiber comprising (a) depositing a synthetic cladding doped with _P2O5 and F in a quartz glass matrix tube to obtain a matching or _recessed ( depressed) cladding structure, (b) under the condition of the tube surface temperature of 1200-1400 °C, the core is formed by depositing an unsintered granular layer, ( _c ) _P2O5 and GeO The concentrations of ₂ were maintained at 0.5-5.0 mol% and 3.0-25.0 mol%, respectively, to obtain tubes containing F-doped cladding and porous soot layers, (d) soaking the tubes containing porous soot layers in 1-2 hours in a solution containing RE salt at a concentration of 0.002-0.25M with or without aluminum salt at a concentration of 0.05-1.25M, (e) at a rate of 10-50cc/min Drain off the solution, (f) dry the porous layer by passing dry nitrogen or any other inert gas through the tube, (g) gradually heat the tube in the range of 600-1100°C in the presence of oxygen , (h) dehydration of the core layer in the tube in the temperature range of 800-1200°C in the presence of excess _Cl2 , (i) dehydration of the core layer in the presence of a mixture of oxygen and helium at a temperature of 1400-1900°C Sintering, (j) collapsing the tube at a temperature of 2000-2300°C to obtain a preform, (k) covering the preform with a quartz tube, (l) drawing the preform into fibers.

The present invention further provides a method of preparing an erbium-doped optical fiber comprising (a) depositing a composite cladding doped with _P2O5 and F in a quartz glass substrate tube to obtain a matched or depressed cladding _type structure , (b) The core is formed by depositing an unsintered granular layer at a tube surface _temperature of 1200–1350 °C, (c) the concentrations of _P2O5 and _GeO2 are kept at 0.5 in said granular layer, respectively - 3.5 mol% and 3.0-20.0 mol% to obtain a tube comprising a cladding and a porous soot layer doped with F, (d) soaking the tube comprising a porous soot layer in the solution for 1-2 hour, the concentration of Er salt in the solution is 0.004-0.20M, and also contains or does not contain the aluminum salt that concentration is 0.05-1.0M, (e) with the speed of 10-30cc/min the solution is drained, (f) The porous layer is dried by passing dry nitrogen or any other inert gas through the tube, (g) gradually heating the tube in the range of 700-1000°C in the presence of oxygen, (h) at 800-1200°C °C temperature range and in the presence of excess _Cl2 to dehydrate the core layer in the tube, (i) sinter the core layer in the presence of a mixture of oxygen and helium at a temperature of 1400-1800 °C, (i) at 2000 Slumping the tube at a temperature of -2300°C to obtain a preform, (k) covering the preform with a quartz tube, (l) drawing the preform into fibers.

The present invention also provides a method for preparing a rare earth-doped optical fiber, wherein, according to the host glass composition and the concentration of RE/Al in the core layer, by controlling the density of the porous soot layer, the soaking time and the period of oxidation, sintering and slumping treatment conditions to change the RE distribution in the core along the transverse direction. The numerical aperture of the fiber is 0.10-0.30, and the RE concentration at the core is kept between 50-6000ppm, together with the change of RE distribution along the radial direction in the doped area, so as to prepare amplifiers and fiber lasers suitable for different purposes and sensor fibers.

In one embodiment of the present invention, the theoretically estimated relative density of the porous soot is 0.30-0.50 to avoid core-cladding interface defects.

In another embodiment of the invention, the _GeCl4 provided during soot deposition is 10-30% lower than that required to obtain the desired NA numerical aperture.

In yet another embodiment of the present invention, the distribution radius of the pump beam is equal to or greater than the distribution radius of the Er ions in the core in order to increase the exposure of all active ions to the pump light.

In yet another embodiment of the invention, the gain obtained in the fiber is relatively high, with NA (numerical aperture) values close to 0.20.

In yet another embodiment of the present invention, the RE salt used is selected from chloride, nitrate or any other salt soluble in the solvent used in the process.

In yet another embodiment of the present invention, the aluminum salt used is selected from chlorides, nitrates or any other salt soluble in the solvent used in the process.

In yet another embodiment of the present invention, the solution of the salt of aluminum and RE is prepared by using a solvent selected from alcohols and water.

In yet another embodiment of the present invention, during oxidation and sintering, the temperature of the core layer is increased in a gradient of 50-200° C., depending on the composition of the core layer and the concentration of Al/RE.

In yet another embodiment of the present invention, the ratio of _O2 and He mixture ranges from 3:1 to 9:1.

In yet another embodiment of the present invention, the source of chlorine is selected from _CCl4 with helium as the carrier gas.

In yet another embodiment of the present invention, the ratio of Cl ₂ :O ₂ is 1.5:1 to 3.5:1, and the dehydration time is 1-2 hours.

In yet another embodiment of the invention, during sintering, the porous core is sintered in the presence of germanium oxide by supplying GeCl ₄ with input oxygen at a temperature of 1200-1400° C. in order to promote germanium oxide The introduction and obtain the appropriate numerical aperture.

In yet another embodiment of the present invention, the method allows the numerical aperture of the fiber to be varied between 0.10-0.30, maintaining the RE concentration at the core between 50-6000 ppm, along with the RE profile in the doped region along the transverse direction Together, it is possible to prepare fibers suitable for any device.

In yet another embodiment of the invention, the device is a multipurpose amplifier, fiber laser, and sensor using optical fibers.

Yet another embodiment of the present invention is a method of controlling the radial Gaussian RE profile in the core for use in a process for preparing a rare earth-doped optical fiber, wherein the process for preparing an optical fiber comprises the steps of:

a) Deposition of _P2O5 and F in a high silica glass matrix tube to obtain _a matched or depressed cladding type structure.

b) At a temperature of 1200-1400° C., deposit a layer of unsintered particles of predetermined composition to form a core, wherein the contents of P ₂ O ₅ and GeO ₂ in the core are 0.5-5.0 mol % and 3.0- 25.0 mole %, and keep the _GeCl4 concentration in the gas phase 10-30% lower than that required to obtain the required NA value of 0.20.

c) The deposition temperature depends on the composition of the soot and the required porosity. The theoretically estimated porosity of 0.3-0.5 was found to be suitable for avoiding core-cladding interface defects and aggregation after impregnation, and for controlling the distribution of RE in the core to have a maximum concentration at the center.

d) Soak the tube containing the porous soot layer in an alcohol/water solution of RECl ₃ /RE(NO ₃ ) ₃ for 1-2 hours at a concentration of 0.002-0.25M, with or without addition of a concentration of 0.05- 1.25M AlCl ₃ /Al(NO ₃ ) ₃ .

e) Drain the solution slowly at a rate of 10-50 cc/min to avoid defects in the porous soot material, especially at the lower end of the tube.

f) The porous layer is fully dried by passing dry nitrogen in the tube and the tube is remounted on the lathe.

g) repeated heating of the RE/Al-containing particle layer in the temperature range of 600-1100 °C (tube surface temperature) in the presence of _O2 + He, wherein the temperature is increased using a gradient of 50-200 °C, so that Chlorides or nitrates of RE/Al present in the layer are oxidized to the corresponding oxides, wherein the ratio of O ₂ to He is 3:1 to 9:1.

h) Dehydration of the particle core layer containing RE at a temperature range of 800-1200°C in the presence of excess chlorine. CCl ₄ is used as the raw material for Cl ₂ and fed with helium as the carrier gas, which is a lighter gas that diffuses through small pores and facilitates the drying process. The ratio of Cl ₂ :O ₂ is 1.5:1 to 3.5:1, and the dehydration time is 1-2 hours.

i) Then, the porous core layer was sintered by heating the tube to a high temperature of 1900 °C in the presence of _O2 and He. From the above-mentioned drying temperature of 800-1200°C, the temperature is gradually increased in a gradient of 50-200°C, depending on the composition of the core layer and the concentration of RE/Al.

j) During sintering, at a temperature of 1200-1400° C., pure GeCl ₄ is fed together with oxygen input in order to sinter the porous layer in a germania-rich atmosphere capable of promoting the incorporation of germania. The flow rate of GeCl ₄ and the number of passes depend on the required NA value in the fiber. Then, the feeding of _GeCl4 was stopped, and the sintering was continued by gradually increasing the temperature until a transparent glass layer was formed.

k) Slumping is performed at high temperature (>2000° C.) 3-4 times through a burner to obtain a solid glass rod called a preform.

l) Wrap the preform with an appropriately sized quartz tube to obtain the proper core-cladding dimensions in the final preform/fiber.

m) Drawing fibers from the preform.

Brief description of the drawings

Figures 1 and 2 represent the Er fluorescence distribution diagrams on the cross-section of the fiber core.

The present invention is further illustrated by means of the following examples, which should not be construed as limiting the scope of the present invention.

Example 1

●At 1855°C, MCVD method is used to deposit F-doped cladding in the quartz tube.

• At a temperature of 1290°C, the unsintered core is deposited. The flow of the carrier gas through the reactant liquid is adjusted so as to obtain the following composition in the deposited soot layer: SiO ₂ =90.2 mol%, P ₂ O ₅ =1.3 mol% and GeO ₂ =8.5 mol%.

• Soak the tube with the deposit in a solution containing 0.025 (M) ErCl ₃ and 0.15 (M) Al(NO ₃ ) ₃ 9H ₂ O for 1 hour and drain the solution slowly.

- Dry by keeping nitrogen flowing through the tube for 10 minutes.

• Oxidation was carried out at 725°C, 825°C and 950°C, using 2 passes through the burner at each temperature and keeping the He/ _O2 ratio constant at 1:5.

●Dehydration treatment is carried out, wherein the treatment temperature is 1010° C., the ratio of Cl ₂ /O ₂ is 2.5:1, and the treatment time is 1 hour and 15 minutes.

• Increase the temperature to 1400°C in 4 steps. From this stage _GeCl4 was added together with the oxygen input, 3 times at a temperature between 1200-1400°C. The tube was further heated in steps up to 1650°C in order to completely sinter the porous soot layer containing Er and Al. During sintering, the _O2 and He flow ratio was 4.5:1.

• In the usual way, slumping is performed in 3 steps.

• Overcladding was performed to reduce the ratio of core to cladding to 3.6:125. The NA value measured in the fiber was 0.204 ± 0.01.

●The ion concentration of Er ⁺³ in the fiber is 950ppm, and its maximum concentration and distribution at the center of the core are shown in Figure 1 of this specification. The Er distribution at the core was measured from the cross-section of the fiber using a fluorescence spectrometer manufactured by: Photonics Resource Facility, 60 St. George Street, Suite No.331, Toronto, Ontario, Canada M5S 1A7.

●The measured gain of the fiber is 35.4dB. The gain was measured at C-DOT, 39 Main Pusa Road, New Delhi-110 005, using their measurement setup.

Example 2

●At 1840°C, MCVD is used to deposit F-doped cladding in the quartz glass tube.

• At a temperature of 1310°C, the unsintered core is deposited. The flow of the carrier gas through the reactant liquid was adjusted so as to obtain the following composition in the deposited soot layer: SiO ₂ =91.6 mol%, P ₂ O ₅ =1.1 mol% and GeO ₂ =7.3 mol%.

● Soak the tube with the deposit in a solution containing 0.015(M) ErCl ₃ , 6H ₂ O and 0.15(M) Al(NO ₃ ) ₃ 9H ₂ O for 1.5 hours, and slowly dissolve the solution Drain.

- Dry by keeping nitrogen flowing through the tube for 10 minutes.

• Oxidation was carried out at 750°C, 800°C and 900°C, using 2 passes through the burner at each temperature and keeping the He/ _O2 ratio constant at 1:5.

●Dehydration treatment is carried out, wherein the treatment temperature is 915° C., the ratio of Cl ₂ /O ₂ is 2.3:1, and the treatment time is 1 hour.

●Raise the temperature to 1200°C in 3 steps. In this stage, _GeCl4 was added together with the oxygen input, once at each temperature of 1200°C, 1300°C and 1400°C. The tube was further heated in steps up to 1610°C in order to completely sinter the porous soot layer containing Er and Al. During sintering, the flow ratio of _O2 and He was 5:1.

• In the usual way, slumping is performed in 3 steps.

• Cladding was performed to reduce the core to cladding ratio to 3.6:125.

●The NA value measured in the fiber is 0.201±0.01.

● The ion concentration of Er ⁺³ in the fiber is 460ppm, its peak is at the center of the core, and a similar distribution is shown in Fig. 1 .

●The measured fiber gain is up to 37dB. This gain was measured by C-DOT, 39 Main Pusa Road, New Delhi-110 005, using their measurement setup.

Example 3

●At 1870°C, MCVD method is used to deposit F-doped cladding in the quartz tube.

• At a temperature of 1250°C, the unsintered core is deposited. The flow of the carrier gas through the reactant liquid was adjusted so as to obtain the following composition in the deposited soot layer: SiO ₂ =89.1 mol%, P ₂ O ₅ =2.3 mol% and GeO ₂ =8.6 mol%.

• Soak the tube with the deposit in an aqueous solution containing 0.07 (M) ErCl ₃ and 0.25 (M) Al(NO ₃ ) ₃ 9H ₂ O for 1 hour and drain the solution slowly.

- Dry by keeping nitrogen flowing through the tube for 10 minutes.

• Oxidation was carried out at 730°C, 820°C and 925°C with 2 passes through the burner at each temperature, keeping the He/ _O2 ratio constant at 1:6.

●Dehydration treatment is carried out, wherein the treatment temperature is 925°C, the ratio of Cl ₂ /O ₂ is 2.3:1, and the treatment time is 1.5 hours.

• Increase the temperature to 1400°C in 4 steps. _GeCl4 was added together with oxygen input, twice at 1200°C and once each at temperatures of 1300°C and 1400°C. The tube was further heated in steps up to 1725°C in order to completely sinter the porous soot layer containing Er and Al. During sintering, the flow ratio of _O2 and He was 4:1.

• In the usual way, slumping is performed in 3 steps.

• Cladding was performed to reduce the core to cladding ratio to 6.5:125. The NA value measured in the fiber was 0.22 ± 0.1.

●The ion concentration of Er ⁺³ in the fiber is 3020ppm, and its peak concentration is at the center of the core, and the distribution of Er at the core is shown in Figure 2. The distribution of Er is measured at the cross section of the fiber using a fluorescence spectrometer. The manufacturer of the spectrofluorometer is: Photonics Resource Facility, 60 St. George Street, Suite No.331, Toronto, Ontario, Canada M5S 1A7.

The main advantages of the present invention are:

1. In the developed fiber, the RE distribution in the doped region is similar to the Gaussian pump beam intensity distribution in the fiber, and as a result, the overlap between the two is significantly improved, thereby increasing the pump conversion efficiency in the fiber.

2. The distribution radius of the pump beam is equal to or larger than that of the RE ions at the core, thereby increasing the chance of all active ions being exposed to the pump light.

3. Depending on the host glass composition and RE/Al concentration in the core layer, the RE distribution at the core along the lateral direction can be varied by controlling the density of the porous soot layer, soaking time, and treatment conditions during oxidation, sintering, and slumping.

4. Change the composition of the core and cladding glass so that the NA value is close to 0.20 and the concentration of Er ³⁺ ions is 100-1500ppm in order to provide an erbium-doped fiber which is suitable for pumping in optical amplifier applications to amplify the input signal , its gain is 10-37dB.

5. Due to the low NA value in the fiber doped with RE mentioned in item 4 above, it is possible to avoid a large-scale change in the composition between the core and the cladding glass, thereby eliminating the possibility of significantly reducing the fiber performance. Such as residual stress and PMD and other issues.

6. The NA values and mode field diameters of the developed fibers mentioned in the aforementioned items 4 and 5 are not significantly different from signal-transporting fibers, and thus are easy to splice. This minimizes the optical loss of the signal transmitted through the fiber.

7. Sintering in a germania-rich atmosphere can facilitate the ingress of germanium oxide at the core and reduce the amount of germanium halide needed to obtain the required NA value during deposition, making the process efficient and economical.

8. The oxidation step prior to drying and sintering the particle layer reduces the possibility of compositional changes due to RE salt evaporation during subsequent processing.

9. The stepped temperature increase during oxidation and sintering prevents the diffusion of RE as well as co-dopants, thereby minimizing the possibility of compositional changes.

10. For the reasons stated in items 8 and 9 above, the efficiency of the introduction of RE in the doped regions is increased, which makes the method more economical.

11. Improvement in process efficiency increases the yield and reproducibility of the process for the reasons stated in items 8-10 above.

12. The concentration of RE at the core is 50-6000ppm, coupled with the change of RE distribution in the doped area, and the NA value is 0.10-0.30, making the prepared fiber suitable for use as amplifiers for different purposes, microlasers and sensor.

Claims (45)

1. A method for preparing an optical fiber doped with rare earth, said method comprising the steps of: a) depositing a synthetic cladding doped with _P2O5 and F in _a quartz glass substrate tube to obtain a matched or depressed cladding type structure, b) forming the core by depositing a layer of unsintered particles at a tube surface temperature of 1200-1400°C, c) maintaining the concentrations of _P2O5 and _GeO2 in the particle layer at _0.5-5.0 mol% and 3.0-25.0 mol%, respectively, in order to obtain a cladding layer containing F doping and a porous soot layer Tube, d) immersing the tube comprising the porous soot layer in a solution having a RE salt concentration of 0.002-0.25M and also containing or not an aluminum salt at a concentration of 0.05-1.25M for 1-2 hours, e) drain the solution at a rate of 10-50cc/min, f) drying the porous layer by passing dry nitrogen or any other inert gas through the tube, g) gradually heating the tube in the presence of oxygen in the range of 600-1100°C, h) dehydrating the core layer in the tube at a temperature range of about 800-1200° C. in the presence of excess Cl ₂ , i) sintering the core layer in the presence of a mixture of oxygen and helium at a temperature of 1400-1900°C, j) Collapsing the tube by the usual method at a temperature of 2000-2300°C to obtain a preform, k) coating the preform with a quartz tube, and l) The fiber is drawn from the preform by conventional methods. 2. The method according to claim 1, wherein the theoretically estimated relative density of the porous soot is 0.30-0.50 to avoid core-cladding interface defects. 3. The method according to claim 1, wherein the RE salt used is selected from chloride, nitrate or any other salt soluble in the solvent used in the method. 4. The method according to claim 1, wherein the aluminum salt used is selected from chlorides, nitrates or any other salt soluble in the solvent used in the method. 5. The method according to claim 1, wherein the solution of aluminum salt and erbium salt is prepared by using a solvent selected from alcohol and water. 6. The method according to claim 1, wherein the ratio of the mixture of _O2 and He is in the range of 3:1 to 9:1. 7. The method according to claim 1, wherein the source of chlorine is _CCl4 and He is used as the carrier gas. 8. The method according to claim 1, wherein the ratio of _Cl2 : _O2 is 1.5:1 to 3.5:1, and the dehydration time is 1-2 hours. 9. The method according to claim 1, wherein, during sintering of the porous core layer, _GeCl4 is supplied together with input oxygen while maintaining a temperature of 1200-1400°C. 10. The method of claim 1, wherein sintering in a germania-rich atmosphere enables the incorporation of more germanium oxide and reduces the amount of germanium halide required during deposition. 11. The method of claim 1, wherein the oxidation step prior to drying and sintering the particle layer reduces the likelihood of compositional changes due to RE salt evaporation during subsequent processing. 12. The method of claim 1, wherein a temperature gradient of 50-200°C during the oxidation and sintering steps prevents RE and co-dopants from diffusing from doped regions, thereby minimizing compositional changes. 13. The method of claim 1, wherein the introduction efficiency of RE in the doped region is increased. 14. An improved method of preparing Er-doped fibers characterized in particular by a distribution of Er ions at the core which is similar to a Gaussian pump beam intensity distribution, said method comprising the steps of: (a) depositing a synthetic cladding doped with _P2O5 and F in _a quartz glass substrate tube to obtain a matched or depressed cladding type structure, (b) forming the core by depositing a layer of unsintered particles at a tube surface temperature of 1200-1350°C, (c) maintain the concentration of _P2O5 and _GeO2 at _0.5-3.5 mol % and 3.0-20.0 mol %, respectively, in the particle layer to obtain cladding and porous soot containing doped F layers of tubes, (d) soaking the tube comprising the porous soot layer in a solution having an Er salt concentration of 0.004-0.20M and also containing or not an aluminum salt at a concentration of 0.05-1.0M for 1-2 hours, (e) drain the solution at a rate of 10-30cc/min, (f) drying the porous layer by passing dry nitrogen or any other inert gas through the tube, (g) gradually heating the tube in the range of 700-1000°C in the presence of oxygen, (h) dehydrating the core layer in the tube at a temperature in the range of 800-1200°C in the presence of excess _Cl2 , (i) sintering the core layer in the presence of a mixture of oxygen and helium at a temperature of 1400-1800°C, (j) Collapsing the tube by the usual method at a temperature of 2000-2300°C to obtain a preform, (k) wrapping the preform with a quartz tube, and (l) Drawing fibers from preforms using conventional methods. 15. The method of claim 14, wherein the theoretically estimated relative density of the porous soot is 0.30-0.50 to avoid core-cladding interface defects. 16. A method according to claim 14, wherein the Er salt used is selected from chloride, nitrate or any other salt soluble in the solvent used in the method. 17. A method according to claim 14, wherein the aluminum salt used is selected from chlorides, nitrates or any other salt soluble in the solvent used in the method. 18. The method according to claim 14, wherein the solution of aluminum salt and erbium salt is prepared by using a solvent selected from alcohol and water. 19. The method according to claim 14, wherein the ratio of the mixture of _O2 and He is in the range of 4:1 to 9:1. 20. The method according to claim 14, wherein the source of chlorine is _CCl4 and He is used as the carrier gas. 21. The method according to claim 14, wherein the ratio of _Cl2 : _O2 is 1.5:1 to 3.5:1, and the dehydration time is 1-2 hours. 22. The method according to claim 14, wherein, during sintering of the porous layer, _GeCl4 is supplied together with input oxygen while maintaining a temperature of 1200-1400°C. 23. The method according to claim 14, wherein the developed fiber has a controllable RE distribution in the doped region similar to the Gaussian pump beam intensity distribution in the fiber, with its maximum concentration at the center, resulting in significantly improved The overlap between the two distributions is reduced, thereby increasing the pump conversion efficiency in the fiber. 24. The method of claim 14, wherein the distribution radius of the pump beam is equal to or greater than that of Er ions at the core, thereby increasing the exposure of all active ions to the pump light. 25. A method according to claim 24, wherein a relatively high gain is obtained in the fiber with an NA value close to 0.20. 26. A method according to claim 14, wherein, due to the low NA value in the fiber, large changes in the composition between the core and the cladding glass are avoided, thereby eliminating e.g. residual stresses that could significantly degrade the properties of the fiber and PMD etc. issues. 27. The method according to claim 14, wherein the composition of the core and cladding glasses is adapted to obtain an NA value of 0.20 and an Er ion concentration of ^100-1500 ppm without aggregation so as to provide EDF pumped to amplify an input signal (with a gain of 10-37dB) in an optical amplifier application. 28. The method of claim 14, wherein the developed fibers have NA values and mode field diameters not significantly different from signal transmission fibers for ease of splicing, which minimizes optical loss of signals transmitted through the fibers. 29. The method of claim 14, wherein sintering in a germania-rich atmosphere reduces the amount of germanium halide required to obtain a desired NA value during deposition. 30. The method of claim 14, wherein the oxidation step prior to drying and sintering the particle layer reduces the likelihood of compositional changes due to Er salt evaporation during subsequent processing. 31. The method of claim 14, wherein a temperature gradient of 50-200°C during the oxidation and sintering steps prevents diffusion of RE and co-dopants, thereby minimizing the possibility of compositional changes. 32. The method of claim 14, wherein the efficiency of introduction of RE in doped regions is increased, which in turn improves the economics and reproducibility of the method. 33. The method according to claim 14, wherein the numerical aperture of the fiber is 0.10-0.30, the concentration of Er at the core is maintained at 50-6000 ppm, and the Er profile in the doped region is added, so that the prepared fiber is suitable for use as Amplifiers, fiber lasers and sensors for different purposes. 34. A method of controlling a radial Gaussian RE profile in a core used in a process for preparing a rare earth-doped optical fiber, wherein said method comprises the steps of: a) forming the core by depositing a layer of unsintered particles at a tube surface temperature of 1200-1400°C, b) maintaining the concentrations of _P2O5 and _GeO2 at _0.5-5.0 mol % and 3.0-25.0 mol %, respectively, in the particle layer to obtain cladding and porous soot layers containing doped F tube, c) soaking the tube comprising the porous soot layer in a solution having a RE salt concentration of 0.002-0.25M and also containing or not an aluminum salt at a concentration of 0.05-1.25M for 1-2 hours, d) drain the solution at a rate of 10-50cc/min, e) drying the porous layer by passing dry nitrogen or any other inert gas through the tube, f) gradually heating the tube in the presence of oxygen in the range of 700-1100°C and increasing the temperature in a gradient of 50-200°C, g) dehydration of the core layer in the tube at a temperature range of 800-1200° C. in the presence of excess Cl ₂ , h) sintering the core layer in a mixture in the presence of oxygen at a temperature of 1400-1900° C., with a temperature increase in a gradient of 50-200° C., i) Collapsing the tube by the usual method at a temperature of 2000-2300°C to obtain a preform, j) Fibers are drawn from the preform using conventional methods. 35. The method of claim 34, wherein the theoretically estimated relative density of the porous soot is 0.30-0.50 to avoid core-cladding interface defects. 36. A method according to claim 34, wherein the RE salt used is selected from chloride, nitrate or any other salt soluble in the solvent used in the method. 37. A method according to claim 34, wherein the aluminum salt used is selected from chlorides, nitrates or any other salt soluble in the solvent used in the method. 38. The method according to claim 34, wherein the solution of aluminum salt and erbium salt is prepared by using a solvent selected from alcohol and water. 39. The method according to claim 34, wherein the ratio of the mixture of _O2 and He is in the range of 3:1 to 9:1. 40. The method according to claim 34, wherein the source of chlorine is _CCl4 and He is used as the carrier gas. 41. The method according to claim 34, wherein the ratio of _Cl2 : _O2 is 1.5:1 to 3.5:1, and the dehydration time is 1-2 hours. 42. The method according to claim 34, wherein, during sintering of the porous layer, _GeCl4 is supplied together with input oxygen while maintaining a temperature of 1200-1400°C. 43. The method of claim 34, wherein the oxidation step prior to drying and sintering the particle layer reduces the likelihood of compositional changes due to Er salt evaporation during subsequent processing. 44. The method of claim 34, wherein the temperature gradient during the oxidation and sintering steps prevents diffusion of RE and co-dopants, which in turn prevents compositional changes. 45. The method according to claim 34, wherein the numerical aperture of the fiber is 0.10-0.30, the concentration of Er at the core is maintained at 50-6000 ppm, and the change of the Er profile in the doped region makes the prepared fiber suitable for use in on any device. 46. The method of claim 34, wherein said devices are amplifiers for different applications, fiber lasers and sensors, and other devices using optical fibers.

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