Process of fabrication of Erbium and Ytterbium-co-doped multi-elements silica glass based cladding-pumped fiber

Abstract

The present application provides a process of fabrication of erbium and ytterbium-co-doped multielements silica glass based cladding-pumped fiber for use as a highly efficient high power optical amplifier.

Claims

1. A process for fabricating erbium and ytterbium-co-doped multi-elements silica glass based cladding-pumped fiber for use as an optical amplifier, the process comprising the steps of: a) depositing a pure SiO.sub.2 synthetic cladding within a silica glass substrate tube to obtain match clad structure; b) forming a porous core layer inside the silica glass substrate tube by depositing a fluorinated-phospho silica porous soot layer using a flow of O.sub.2/POCl.sub.3 in a range of 800 to 900 sccm and SF.sub.6 in a range of 25 to 35 sccm, at a temperature in a range of 1420-1455° C. and maintaining a burner speed a range of 28-35 mm/min; c) pre-sintering the deposited fluorinated-phospho silica porous soot layer at a temperature in a range of 1075-1125° C. by keeping the burner speed in a range of 305-315 mm/min in a forward direction to maintain a uniform porosity; d) soaking the pre-sintered fluorinated-phospho silica porous soot layer into an alcoholic solution for one hour, wherein the alcoholic solution comprises AlCl.sub.3 6H.sub.2O in a range of 0.12 to 0.15 (M), ErCl.sub.3 6H.sub.2O in a range of 0.02 to 0.023(M), YbCl.sub.3 6H.sub.2O in a range of 0.25 to 0.28 (M), CeCl.sub.3 7H.sub.2O in a range of 0.003 to 0.0035(M) and H.sub.3BO.sub.3 in a range of 0.5 to 0.75(M); e) drying the soaked fluorinated-phospho silica porous soot layer under flow of an inert Ar gas for 30 to 45 minutes; f) incorporating Er.sub.2O.sub.3, Yb.sub.2O.sub.3, P.sub.2O.sub.5, F, CeO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3 into the dried fluorinated-phospho silica porous soot layer in the presence of O.sub.2 and He at a temperature within a range of 700-850° C. for efficient oxidation to form the core layer; g) dehydrating the core layer at a temperature in a range of 850 to 900° C. in presence of Cl.sub.2 and O.sub.2 to form a porous dried core layer; h) sintering the porous core layer in presence of a mixture of O.sub.2 and He at a temperature in a range of 1350 to 1890° C. with flow of O.sub.2/GeCl.sub.4 and O.sub.2/POCl.sub.3 to form a final core layer; i) repeating steps (b) to (h) 2-3 times to form the final core layer of a thickness in a range of 10.0-11.5 μm; j) collapsing the silica glass substrate tube having the final core layer at a temperature in a range of 2000 to 2250° C. with the flow of POCl.sub.3 to obtain an Er/Yb codoped multi-elements silica glass based optical preform of 10.0±0.1 mm diameter; k) increasing the preform diameter from 10.0±0.1 mm to 18.5±0.1 mm through an overcladding process using a thick silica tube at a temperature in a range of 2200 to 2400° C. to form an overcladded preform; l) grinding the overcladded preform to a grinding length in a range of 0.72-0.75 mm from a periphery of the overcladded preform at each point of eight positions separated by equal distances followed by polishing of each grinded sides to form an octagonal shaped preform; and m) drawing an octagonal cladding shaped Er/Yb doped fiber from the octagonal shaped preform using a PC 375L AP compound as a low RI resin coating at a preform feed down speed in a range of 0.6 to 0.7 mm/min and at a fiber drawing speed in a range of 20-25 m/min.

2. The process as claimed in claim 1, wherein dehydrating the core layer in presence of Cl.sub.2 and O.sub.2 comprises dehydrating with 2:1 to 2.5: 1 of Cl.sub.2 and O.sub.2, and wherein sintering the porous dried core layer in presence of a mixture of O.sub.2 and He comprises sintering the porous dried core layer with He: O.sub.2 in a ratio of 0.5dehydrating the core layer in presence of Cl.sub.2 and O.sub.2 comprises dehydrating with 2:1 to 2.5: 1 of Cl.sub.2 and O.sub.2, and wherein sintering the porous dried core layer in presence of a mixture of O.sub.2 and He comprises sintering the porous dried core layer with He: O.sub.2 in a ratio of 0.5.

3. The process as claimed in claim 1, wherein sintering the porous dried core layer in presence of a mixture of O.sub.2 and He with flow of O.sub.2/GeCl.sub.4 and O.sub.2/POCl.sub.3 comprises sintering with the flow of O.sub.2/GeCl.sub.4 in a range of 5 to 10 sccm and with the flow of O.sub.2/POCl.sub.3 in a range of 10 to 15 sccm to form a transparent glass layer.

4. The process as claimed in claim 1, wherein collapsing the silica glass substrate tube having the final core layer comprises collapsing with the flow of O.sub.2/POCl.sub.3 in a range of 4.0 to 7.0 sccm, and wherein one end of the silica glass substrate tube is nearly closed to avoid central dip formation in refractive index profile (RIP).

5. The process as claimed in claim 1, wherein increasing the preform diameter from 10.0±0.1 mm to 18.5±0.1 mm through the overcladding process using a thick silica tube comprises using the thick silica tube having a dimension of OD/ID of 20/12 mm to maintain a core diameter of the octagonal cladding shaped Er/Yb doped fiber in a range of 10.0 to 11.81 micron.

6. The process as claimed in claim 1, wherein incorporating Er.sub.2O.sub.3, Yb.sub.2O.sub.3, P.sub.2O.sub.5, F, CeO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3 comprises incorporating multi-elements Er, Yb, P, F, Ce, Al and B uniformly along a diameter of an octagonal classing shaped fiber.

7. The process as claimed in claim 1, wherein the octagonal cladding shaped Er/Yb doped fiber has a numerical aperture (NA) between 0.20±0.01 to 0.24±0.01 and wherein the octagonal cladding shaped Er/Yb doped fiber has a cladding absorption loss in a range from 1.9 to 2.8 dB/m at 915 nm and a core absorption loss in a range from 37 to 52 dB/m at 1530 nm.

8. The process as claimed in claim 1, wherein the incorporating Er.sub.2O.sub.3, Yb.sub.2O.sub.3, P.sub.2O.sub.5, F, CeO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3 comprises doping with GeO.sub.2 in a range of 1.5 to 2.5 wt %, CeO.sub.2 in a range of 0.12 to 0.191 wt %, P.sub.2O.sub.5 in a range of 22.0 to 23.85 wt %, B.sub.2O.sub.3 in a range of 0.95 to 2.51 wt %, F in a range of 0.025 to 0.030 wt %, Al.sub.2O.sub.3 in a range of 0.356 to 0.525 wt %, Er.sub.2O.sub.3 in a range of 0.41 to 0.605 wt %, Yb.sub.2O.sub.3 in a range of 4.54 to 5.10 wt % and Yb/Er ratio in a range of 7.5 to 11.07.

9. The process as claimed in claim 8, wherein doping B.sub.2O.sub.3 along with P.sub.2O.sub.5 reduces fluorescence life time of .sup.4I.sub.11/2 level of Er while increasing the fluorescence life time of .sup.4I.sub.13/2 level of Er; and wherein Yb/Er ratio in a range of 7.5 to 11.07 suppresses Yb self lasing at 1.0 micron intrinsically.

10. The process as claimed in claim 8, wherein the erbium and ytterbium (Er/Yb) co-doped multi-elements silica glass based cladding shaped fibers are configured to be used as fiber lasers and high power optical amplifiers generating 5.35 to 20 W power with lasing efficiency above >38%.

11. The process as claimed in claim 8, wherein the erbium and ytterbium co-doped multi-elements silica glass based cladding shaped fiber is suitable for use in a radiation environment exhibiting <10% degradation of output power under low dose rates of .sup.60Co gamma radiation up to 6.0 Krad, wherein dose rate varies from 0.35 to 0.70 rad/s, and wherein the dose rate is equivalent to dose rates in free space.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is illustrated in FIGS. 1 to 10 of the drawings accompanying this specification.

(2) FIG. 1 represents the axial view of Octagonal shaped preform made from circular pristine preform through grinding of 0.75 mm length from the periphery of circular overcladded preform of 18.6 mm diameter at each point of eight positions separated by equal distances mentioned in Example-3 in accordance with an embodiment of the present invention.

(3) FIG. 2 represents a cross sectional view of fabricated octagonal cladding shaped fiber having average 11.81 micron core diameter mentioned in Example-3 in accordance with an embodiment of the present invention.

(4) FIG. 3 represents the elemental distribution profile along the core diameter measured from the electron probe micro analysis (EPMA) data of fiber sample showing uniform distribution of all the multielements along the core diameter mentioned in Example-3 in accordance with an embodiment of the present invention.

(5) FIG. 4 represents the loss curve showing core absorption 52.0 dB/m at 1530 nm of fiber sample mentioned in Example-3 in accordance with an embodiment of the present invention.

(6) FIG. 5 represents the loss curve showing cladding absorption 2.6 dB/m at 915 nm of fiber sample mentioned in Example-3 in accordance with an embodiment of the present invention.

(7) FIG. 6 represents the refractive index profile (RIP) of the fiber sample showing numerical aperture 0.24±0.01 measured by Fiber Analyzer mentioned in Example-3 in accordance with an embodiment of the present invention.

(8) FIG. 7 represents the efficiency curve of output power at 1558 nm wavelength using a 6.5 meter length of fiber sample under pumping at 940 nm showing 38.5% efficiency mentioned in Example-3 in accordance with an embodiment of the present invention.

(9) FIG. 8 represents the lasing output spectra under 52.5 W launched pump power at 940 nm using 6.5 meter of fiber sample showing 20.0 W output power mentioned in Example-3 in accordance with an embodiment of the present invention.

(10) FIG. 9 represents 1 μm Yb ASE at 20.0 W output power level of fiber sample showing no 1.0 micron Yb.sup.3+ self lasing mentioned in Example-3 in accordance with an embodiment of the present invention.

(11) FIG. 10 represents the normalized output power of Er/Yb codoped fiber against on-line .sup.60Co-gamma radiation having dose rate of 0.35 rad/sec showing 8.0% degradation of output power mentioned in Example-3 in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(12) Accordingly the present invention provides a process of fabrication of erbium and ytterbium-co-doped multi-elements silica glass based cladding-pumped fiber for use as a highly efficient high power optical amplifier, comprising the following steps:

(13) (a) deposition of pure SiO.sub.2 doped synthetic cladding within a silica glass substrate tube to obtain matched clad type structure; (b) forming a core by depositing fluorinated-phospho silica porous soot layer under flow of O.sub.2/POCl.sub.3 in the range of 800-900 sccm and flow of SF.sub.6 in the range of 25-35 sccm at a tube surface temperature in the optimized range of 1420−1455° C. following back-pass deposition technique where burner speed maintained within 28-35 mm/min;

(14) (c) pre-sintering of the porous soot layer at optimized temperature in the range of 1075-1125° C. to maintain uniform porosity keeping the burner speed within 305-315 mm/min in forward direction; (d) soaking the porous fluorinated phospho silica layer for one hour into an alcoholic solution maintaining the strength of AlCl.sub.3 6H.sub.2O in the range of 0.12 to 0.15(M), ErCl.sub.3 6H.sub.2O in the range of 0.02 to 0.023(M), YbCl.sub.3 6H.sub.2O in the range of 0.25 to 0.28 (M), CeCl.sub.3 7H.sub.2O in the range of 0.003 to 0.0035(M) and strength of H.sub.3BO.sub.3 in the range of 0.5 to 0.75(M); (e) drying of the soaked core layer under flow of Ar inert gas for 30 to 45 minutes followed by oxidation in the presence of O.sub.2 and He around 700-850° C. in a stepwise manner maintaining a constant He/O.sub.2 ratio of 0.5; (f) dehydrating the soaked layer around 850-900 C with flow of He, Cl.sub.2 and O.sub.2 maintaining the ratio of Cl.sub.2/O.sub.2 2.0 to 2.5; (g) sintering the porous layer in presence of a mixture of O.sub.2 and He within 1350 to 1890° C. temperature range along with flow of O.sub.2/GeCl.sub.4 in the range of 5 to 10 sccm and O.sub.2/POCl.sub.3 in the range of 10 to 15 sccm; (h) repeating of the process steps from b) to g) 2 to 3 times to achieve desired core thickness of 10.0-11.5 μm with respect to 125 μm cladding diameter in fiber stage drawn from the overcladded preform; (i) collapsing the tube at a temperature above 2000° C. with flow of O.sub.2/POCl.sub.3 in the range of 4.0 to 7.0 sccm to obtain final preform; (j) jacketing of the preform with silica tubes of 20/12 OD/ID thick silica tubes to achieve preform diameter around 18.4-18.6 mm through the overcladding process using a thick silica tube at a temperature in the range of 2200 to 2400° C.; (k) grinding of the preform under optimized grinding length within 0.72-0.75 mm from the periphery of circular overcladded preform at each point of eight positions separated by equal distances followed by polishing of each grinded sides to an octagonal shape; (1) drawing of octagonal cladding shaped fibers maintaining optimized condition such as preform feed-down rate within 0.6 to 0.7 mm/min and drawing speed within 20-25 m/min from the octagonal shaped preform using PC 375 L AP as low RI resin; (m) the doping levels of different elements in multielements silica glass core of fiber maintaining P.sub.2O.sub.5 concentrations between 22.0 to 23.85 wt % in the ultimate core layer; (n) maintaining GeO.sub.2 concentrations between 1.5 and 2.5 wt % in the core layer, when added with P.sub.2O.sub.5; (o) maintaining F concentrations between 0.025 and 0.030 wt % in the core layer, when added with P.sub.2O.sub.5; (p) soaking the tube containing the porous soot layer into a solution containing RE, Al and Ce as their chloride salts while B in form of boric acid; maintaining Al.sub.2O.sub.3 concentrations between 0.356 to 0.525 wt %, B.sub.2O.sub.3 concentrations between 0.95 to 2.51 wt %, CeO.sub.2 concentration within 0.12 to 0.191 wt %, Er.sub.2O.sub.3 concentration within 0.41 to 0.605 wt % and Yb.sub.2O.sub.3 concentration within 4.54 to 5.1 wt %; (q) maintaining Yb to Er ratio from 7 to 11; (r) measurement of lasing performance under 940 nm laser diode pumping; (s) On-line measurement of the stability of lasing output power under variation of the radiation dose rates of .sup.60Co gamma radiation from 0.35 to 0.70 rad/s (equivalent to dose rates in free space) upto cumulative dose of 6.0 Krad.

(15) In the present invention, we propose the use of multielements silica glass as a single complete solution providing low to moderately high output power (5 to 20 W) with good lasing efficiency (>38.0%) without self lasing around 1.0 micron with good stability of output power under .sup.60Co gamma-radiation environment having degradation of lasing output power (<10.0% of the initial power) as follows.

(16) In an embodiment of the present invention the PD resistivity in Er/Yb codoped multielement silica glass based fibers can be greatly improved due to presence of Ce to the core glass composition where each valence state of Ce ions can reduce the number of color centers effectively by capturing both hole and electron related colour centers for enhancement of the radiation resistance behavior of the fiber in ionizing radiation environment. On the other hand, P.sub.2O.sub.5 doping levels of 22.0 to 23.85 wt % along with minor doping of B.sub.2O.sub.3 in multielements silica glass can transfer the energy effectively from excited Yb ions to Er ions as the glass reduces the fluorescence life time of .sup.4I.sub.11/2 level of Er with increasing the fluorescence life time of .sup.4I.sub.13/2 level of Er to get high lasing efficiency above 38.0% with maximum output power of 20.0 W. On the other hand, the suitable doping levels of Yb within 4.54-5.1 wt % and Er within 0.41-0.605 wt % along with their ratio within 7.5 to 11.0 in this multielement silica glass suppress the 1.0 micron Yb.sup.3+ self lasing intrinsically which provide also the high lasing efficiency above 38% with maximum 20.0 W output power. The present invention is illustrated in FIGS. 1 to 10 of the drawing accompanying this specification.

(17) In another embodiment of the present invention, the Octagonal shaping of the overcladded preform by proper grinding process through milling of the silica cladding material varying the grinding length 0.72 to 0.75 mm from the periphery of circular overcladded preform at each point of eight positions separated by equal distances followed by polishing of each grinding sides was made in order to increase the pump absorption efficiency. In FIG. 1 described the axial view of one octagonal shaped preform made from circular pristine preform through grinding of 0.75 mm length from the periphery of circular overcladded preform of 18.6 mm diameter at each point of eight positions separated by equal distances in accordance with an embodiment of the present invention. The Octagonal cladding shaped fiber drawn from such octagonal shaped preform under optimized preform feed down and fiber drawing speed. In FIG. 2 described the cross sectional view of fabricated octagonal cladding shaped fiber having an average 11.81 micron core diameter of one of the fibers in accordance with an embodiment of the present invention. The present invention provide the optimized process parameters for uniform doping of all the elements along the core diameter of one fiber for achieving good lasing efficiency above 38.0%. In FIG. 3 described the elemental distribution profile along the core diameter measured from the electron probe micro analysis (EPMA) data of fiber sample showing uniform distribution of all the multielements along the core diameter in accordance with an embodiment of the present invention. In the present invention, the suitable doping levels of Er and Yb ions in multielement doped silica glass based optical fibers showing core absorption losses within 1.9-2.8 dB/m at 915 nm and cladding absorption losses within 37.0-52.0 dB/m at 1530 nm exhibit good lasing efficiency above 38.0% for getting 5.0 to 20.0 W output power with suppression 1.0 micron Yb.sup.3+ self lasing. In FIGS. 4 and 5 described the core absorption loss of 52.0 dB/m at 1530 nm and cladding absorption loss of 2.6 dB/m at 915 nm of one of the developed fibers in accordance with an embodiment of the present invention. In FIG. 6 described the refractive index profile (RIP) of the fiber sample showing numerical aperture 0.24±0.01 measured by Fiber Analyzer mentioned in accordance with an embodiment of the present invention. In FIG. 7 described the lasing efficiency at 1558 nm wavelength using a 6.5 meter length of fiber sample under pumping at 940 nm showing 38.5% efficiency in accordance with an embodiment of the present invention. In FIG. 8 described the lasing output spectra under 52.5 W launched pump power at 940 nm using 6.5 meter of fiber sample showing 20.0 W output power in accordance with an embodiment of the present invention. In FIG. 9 described 1 μm Yb ASE at 20.0 W output power level of fiber sample showing no 1.0 micron Yb.sup.3+ self lasing in accordance with an embodiment of the present invention. In FIG. 10 described the normalized output power of Er/Yb codoped fiber against on-line .sup.60Co-gamma radiation having dose rate of 0.35 rad/sec showing 8.0% degradation of output power which makes this fiber suitable for high power optical amplifier application in CATV, free space communication and FTTH in accordance with an embodiment of the present invention.

(18) The use of multielements silica glass as an alternate solution to overcome the above mentioned problems. The novelty of fabricated fiber is use of multielement glass of suitable composition where F doping helps to achieve radiation hardness by elimination Si and P-related defect center, Ce-doping helps to prevent the formation of Yb-related defect centers, P doping enhances Er/Yb solubility in glass and Al doping levels require optimization to minimize the formation of Al—O hole defect centers, Ge doping helps to control numerical aperture (0.19 to 0.25) while B along with P doping enhance the energy transfer efficiency from Yb to Er by reducing the fluorescence life time of .sup.4I.sub.11/2 level of Er while increasing the fluorescence life time of .sup.4I.sub.13/2 level of Er to suppress 1.0 micron Yb self lasing intrinsically, which makes this fiber suitable for high power optical amplifier application having output power 5.0 W (+37 dBm)−20.0 W (+43 dBm) showing <10% degradation of lasing output power under cumulative radiation dose of .sup.60Co gamma radiation up to 6.0 Krad at low dose rates (˜0.35 to 0.70 rad/s) equivalent to dose rates in free space radiation.

(19) Deposition of porous core layer of suitable composition containing SiO.sub.2—P.sub.2O.sub.5—F employing backpass deposition technique at suitable deposition temperature followed by pre-sintering of porous soot layer prior to solution doping maintaining suitable burner speed and temperature; impregnation of porous soot layer into an alcoholic solution comprising suitable dopants like Al, Yb, Er, Ce, and B for definite time span followed by drying, oxidation and sintering along with repetition of above mentioned steps 2-3 times; collapsing of the tube by reducing one end of the tube very close to sealing to avoid evaporation of different dopants; overcladding of the initial preform made by the above steps; grinding and shaping of the fabricated preform to octagonal shape.

(20) The following examples are given by way of illustration of the working of the invention in actual practice and therefore should not be construed to limit the scope of the present invention.

Example 1

(21) Deposition of pure SiO.sub.2 layers inside a silica glass substrate tube employing MCVD process at a temperature of 1850° C. to achieve match clad geometry.

(22) Unsintered core deposition at a temperature of 1430±5° C. using backpass deposition technique maintaining burner speed of 30 mm/min comprising P.sub.2O.sub.5 and F with flow of 800 sccm O.sub.2/POCl.sub.3 and 25.0 sccm SF.sub.6.

(23) Pre-sintering of the porous soot layer was performed at a temperature of 1120±5° C. maintaining burner speed of 315 mm/min to maintain uniform porosity.

(24) The porous soot layer was dipped in an alcoholic soaking solution containing 0.12 (M) AlCl.sub.3 6H.sub.2O, 0.02(M) ErCl.sub.3 6H.sub.2O, 0.25(M) YbCl.sub.3 6H.sub.2O, 0.003(M) CeCl.sub.3 7H.sub.2O and 0.5(M) H.sub.3BO.sub.3 as chloride precursor of Al, Er, Yb and Ce in addition to Boric acid as a source of B. The composition was selected to achieve Al.sub.2O.sub.3 concentration of 0.356 wt %, Er.sub.2O.sub.3 concentration of 0.41 wt %, Yb.sub.2O.sub.3 concentration of 4.54 wt %, the CeO.sub.2 concentration of 0.12 wt % and B.sub.2O.sub.3 0.95 wt % along with with Yb to Er ratio of 11.07 determined from electron prove microanalyses (EPMA).

(25) After draining out of the solution, the soaked layer was dried with flow of inert Ar gas for 30 minutes.

(26) Remounting the soaked tube in MCVD lathe and oxidation was performed in the presence of O.sub.2 and He around 700-850° C. in a stepwise manner maintaining a constant He/O.sub.2 ratio of 0.5.

(27) Dehydration was carried out at a temperature of 900° C. with a Cl.sub.2:O.sub.2 ratio of 2.5:1.

(28) Sintering of the core layer was performed in the presence of a mixture of O.sub.2 and He in the temperature range of 1350 to 1850° C. along with flow of O.sub.2/GeCl.sub.4 at the rate of 5.0 sccm and O.sub.2/POCl.sub.3 at 10.0 sccm for conversion of the soaked deposited multielements doped porous layer into transparent glass layer smoothly.

(29) The vapor phase composition of porous layer as well as flow of O.sub.2/GeCl.sub.4 and O.sub.2/POCl.sub.3 was adjusted such that final preform core comprised of 22 wt % P.sub.2O.sub.5, 1.5 wt % GeO.sub.2 and 0.025 wt % F.

(30) To achieve the desired core thickness starting from deposition of porous soot layer up to the sintering step was repeated two times to achieve the final core thickness of 10 μm in ultimate fiber.

(31) Collapsing of the tube in 2 steps at a temperature in the range of 2000−2150° C. to obtain solid rod called preform in the presence of O.sub.2/POCl.sub.3 flow at the rate of 4.0 sccm with one end of tube nearly closed to avoid central dip formation significantly in their RI profile.

(32) Jacketing the preform with silica tubes of OD/ID: 20/12 mm dimensions was performed to get final preform of 18.5 mm diameter.

(33) Grinding of 0.73 mm length from the surface of circular overcladded preform along each of eight point on equal distance followed by polishing of the each grinded side into Octagonal shaped structure.

(34) Drawing of fibers with preform feed rate of 0.6 mm/min with drawing speed of 20 m/min from the octagonal shaped preform using PC 375 L AP low RI resin was performed to achieve the final fiber of core diameter 10 μm, with numerical aperture 0.20±0.01.

(35) The absorption of the fabricated fiber was measured which exhibited cladding absorption of 1.9 dB/m at 915 nm and core absorption of 37 dB/m at 1530 nm.

(36) The fabricated fiber exhibited 5.35 W output power with lasing efficiency of 38.2% without showing any Yb self lasing at 1 μm.

(37) The fabricated fiber suppress the degradation of lasing output power (8.8% of the initial power) under .sup.60Co-gamma irradiation upto cumulative radiation of 6.0 Krad under dose rates of 0.70 rad/s (Dose rate equivalent to free space).

Example 2

(38) Deposition of pure SiO.sub.2 layers inside a silica glass substrate tube employing MCVD process at a temperature of 1850° C. to achieve match clad geometry.

(39) Unsintered core deposition at a temperature of 1450±5° C. using backpass deposition technique maintaining burner speed of 35 mm/min comprising P.sub.2O.sub.5 and F with flow of 875 sccm O.sub.2/POCl.sub.3 and 32.0 sccm SF.sub.6.

(40) Pre-sintering of the porous soot layer was performed at a temperature of 1100±5° C. maintaining burner speed of 305 mm/min to maintain uniform porosity.

(41) The porous soot layer was dipped in an alcoholic soaking solution containing 0.15 (M) AlCl.sub.3 6H.sub.2O, 0.023(M) ErCl.sub.3 6H.sub.2O, 0.25(M) YbCl.sub.3 6H.sub.2O, 0.0032(M) CeCl.sub.3 7H.sub.2O and 0.6(M) H.sub.3BO.sub.3 as chloride precursor of Al, Er, Yb and Ce in addition to Boric acid as a source of B. The composition was selected to achieve Al.sub.2O.sub.3 concentration of 0.525 wt %, Er.sub.2O.sub.3 concentration of 0.605 wt %, Yb.sub.2O.sub.3 concentration of 4.54 wt %, CeO.sub.2 concentration of 0.131 wt % and B.sub.2O.sub.3 1.12 wt % along with with Yb to Er ratio of 7.5 determined from electron prove microanalyses (EPMA).

(42) Remounting the soaked tube in MCVD lathe and oxidation was performed in presence of O.sub.2 and He around 750-825° C. in a stepwise manner maintaining a constant He/O.sub.2 ratio of 0.5.

(43) Dehydration was carried out at a temperature of 875° C. with a Cl.sub.2:O.sub.2 ratio of 2:1.

(44) Sintering of the core layer was performed in the presence of a mixture of O.sub.2 and He in the temperature range of 1350 to 1875° C. along with flow of O.sub.2/GeCl.sub.4 at the rate of 10.0 sccm and O.sub.2/POCl.sub.3 at 12.0 sccm for conversion of the soaked deposited multielements doped porous layer into a transparent glass layer smoothly.

(45) The vapor phase composition of porous layer as well as flow of O.sub.2/GeCl.sub.4 and O.sub.2/POCl.sub.3 was adjusted such that final preform core comprised of 23 wt % P.sub.2O.sub.5, 2.5 wt % GeO.sub.2 and 0.028 wt % F.

(46) To achieve the desired core thickness starting from deposition of porous soot layer up to the oxidation of the soaked soot layer was repeated three times to achieve the final core thickness of 11.4 μm in ultimate fiber.

(47) Collapsing of the tube in 3 steps at a temperature in the range of 2000−2200° C. to obtain solid rod called preform in the presence of O.sub.2/POCl.sub.3 flow at the rate of 5.0 sccm with one end of tube nearly closed to avoid central dip formation significantly in their RI profile.

(48) Jacketing the preform with silica tube of OD/ID: 20/12 mm dimensions was performed to get final preform of 18.4 mm diameter.

(49) Grinding of 0.72 mm length from the surface of circular overcladded preform along each of eight points on equal distance followed by polishing of the each grinded side into Octagonal shaped structure.

(50) Drawing of fibers with preform feed rate of 0.7 mm/min with drawing speed of 25 m/min from the octagonal shaped preform using PC 375 L AP low RI resin was performed to achieve the final fiber of core diameter 11.4 μm, with numerical aperture 0.23±0.01.

(51) The absorption of the fabricated fiber was measured which exhibited cladding absorption of 2.8 dB/m at 915 nm and core absorption of 50 dB/m at 1530 nm.

(52) The fabricated fiber exhibited 11.4 W output power with lasing efficiency of 40.7%. Fiber exhibited ASE without any Yb self lasing at 1 μm.

(53) The fabricated fiber suppress the degradation of lasing output power (5.1% of the initial power) under .sup.60Co-gamma irradiation upto cumulative radiation of 6.0 Krad under dose rates of 0.35 rad/s (Dose rate equivalent to free space).

Example 3

(54) Deposition of pure SiO.sub.2 layers inside a silica glass substrate tube employing MCVD process at a temperature of 1870° C. to achieve match clad geometry.

(55) Unsintered core deposition at a temperature of 1425±5° C. using backpass deposition technique maintaining burner speed of 28 mm/min comprising P.sub.2O.sub.5 and F with flow of 900 sccm O.sub.2/POCl.sub.3 and 35.0 sccm SF.sub.6.

(56) Pre-sintering of the porous soot layer was performed at a temperature of 1080±5° C. maintaining burner speed of 310 mm/min to maintain uniform porosity.

(57) The porous soot layer was dipped in an alcoholic solution containing 0.14 (M) AlCl.sub.3 6H.sub.2O, 0.022(M) ErCl.sub.3 6H.sub.2O, 0.28(M) YbCl.sub.3 6H.sub.2O, 0.0035(M) CeCl.sub.3 7H.sub.2O and 0.75(M) H.sub.3BO.sub.3 as chloride precursor of Al, Er, Yb and Ce in addition to Boric acid as a source of B. The composition was selected to achieve Al.sub.2O.sub.3 concentration of 0.50 wt %, Er.sub.2O.sub.3 concentration of 0.50 wt %, Yb.sub.2O.sub.3 concentration of 5.10 wt %, CeO.sub.2 concentration of 0.191 wt % and B.sub.2O.sub.3 2.51 wt % along with with Yb to Er ratio of 10.17 determined from electron prove microanalyses (EPMA).

(58) Unsintered core deposition at a temperature of 1425±5° C. using backpass deposition technique maintaining burner speed at 28 mm/min comprising P.sub.2O.sub.5 along with GeO.sub.2 and F. The vapor phase composition of porous layer was adjusted such that final preform core comprised of P.sub.2O.sub.5 concentrations of 23.85 wt %, GeO.sub.2 concentrations 2.43 wt % and F concentrations around 0.030 wt %.

(59) Pre-sintering of the porous soot layer was performed at a temperature of 1080±5° C. maintaining burner speed of 310 mm/min to maintain uniform porosity.

(60) Remounting the soaked tube in MCVD lathe and oxidation was performed in the presence of O.sub.2 and He around 700-825° C. in a stepwise manner maintaining a constant He/O.sub.2 ratio of 0.5.

(61) Dehydration was carried out at a temperature of 850° C. with a Cl.sub.2:O.sub.2 ratio of 2:1.

(62) To achieve the desired core thickness starting from deposition of porous soot layer up to the oxidation of the soaked soot layer was repeated 3 times to achieve the final core thickness of 11.81 m in ultimate fiber.

(63) Sintering of the core layer was performed in the presence of a mixture of O.sub.2 and He in the temperature range of 1370 to 1890° C. along with flow of O.sub.2/GeCl.sub.4 at the rate of 7.0 sccm and O.sub.2/POCl.sub.3 at 15.0 sccm for conversion of the soaked deposited multielements doped porous layer into a transparent glass layer smoothly.

(64) The vapor phase composition of porous layer as well as flow of O.sub.2/GeCl.sub.4 and O.sub.2/POCl.sub.3 was adjusted such that final preform core comprised of 23.85 wt % P.sub.2O.sub.5, 2.43 wt % GeO.sub.2 and 0.030 wt % F.

(65) Collapsing of the tube in 3 steps at a temperature in the range of 2000−2250° C. to obtain solid rod called preform in the presence of POCl.sub.3 flow with one end of tube nearly closed to avoid P.sub.2O.sub.5 and GeO.sub.2 evaporation.

(66) Jacketing the preform with silica tube of OD/ID: 20/12 mm dimensions was performed prior to grinding of the preform followed by polishing to octagonal shape of final diameter around 18.6 mm.

(67) Grinding of 0.75 mm length from the surface of circular overcladded preform along each of 6 points on equal distance followed by polishing of each grinded side into Octagonal shaped structure.

(68) Drawing of fibers with preform feed rate of 0.65 mm/min with drawing speed of 20 m/min from the octagonal shaped preform using low RI resin was performed to achieve the final fiber of core diameter 11.81 μm, with numerical aperture 0.24±0.01.

(69) The absorption of the fabricated fiber was measured which exhibited cladding absorption of 2.6 dB/m at 915 nm and core absorption of 52 dB/m at 1530 nm.

(70) The fabricated fiber exhibited 20 W output power with lasing efficiency of 38.5%. Fiber exhibited ASE without any Yb self lasing at 1.0 μm.

(71) The fabricated fiber suppress the degradation of lasing output power (8.0% of the initial power) under .sup.60Co-gamma irradiation upto cumulative radiation of 6.0 Krad under dose rates of 0.54 rad/s (Dose rate equivalent to free space).

Advantages of Present Invention

(72) The main advantages of the present invention are: i) Er/Yb co-doped multielements silica glass based cladding pump octagonal shaped low RI coated fiber made by MCVD process in combination with solution doping technique provide output power up to 20.0 W with lasing efficiency >38.0% without Yb.sup.3+ self lasing around 1 μm. ii) Uniform doping of different multielements such as Er, Yb, P, Al, Ce, Ge, B and F along the fiber core diameter through the present invention could be maintained with uniform RI and minimized the central dip formation which reduces the clustering behavior of rare earth ions and enhanced energy transfer from Yb to Er ions to achieve high output power with good lasing efficiency. iii) Sintering of the multielements doped porous core layer with flow of O.sub.2/GeCl.sub.4 and O.sub.2/POCl.sub.3 for conversion of the soaked deposited multielements doped porous layer into transparent glass layer smoothly which provide to make bubble free core glass. iv) Self lasing at 1.0 μm is eliminated by using multielements glass composition achieving maximum output power of 20 W by optimization of the doping levels of P.sub.2O.sub.5, B.sub.2O.sub.3, CeO.sub.2, Er.sub.2O.sub.3 and Yb.sub.2O.sub.3 along with the Yb/Er ratio within 7.5 to 11 for efficient energy transfer from Yb to Er ions. v) Suppression of the Photodarkening phenomenon has been achieved through optimization of the doping levels of Al.sub.2O.sub.3 along with CeO.sub.2 with respect to the doping levels of Er.sub.2O.sub.3 and Yb.sub.2O.sub.3 by minimizing the formation of Yb.sup.2+ ion and Al—O hole centers. vi) The radiation resistance behavior of the developed Er/Yb doped fiber (degradation of output power <10% of the initial power) suitable for free space application as investigated using .sup.60Co gamma radiation under low dose rate equivalent to the free space radiation achieved through optimization of the doping levels of F, CeO.sub.2 along with B.sub.2O.sub.3. vii) The developed Er/Yb codoped cladding pump optical fiber based on multielements silica glass will be very useful for making of high power optical amplifier to be used efficiently in free space communication systems, Fiber to the Home (FTTH) and CATV applications