Indian Patents. 259783:" INTRINSICALLY GAIN FLATTENED ERBIUM DOPED FIBER AMPLIFIER"

Title of Invention	" INTRINSICALLY GAIN FLATTENED ERBIUM DOPED FIBER AMPLIFIER"
Abstract	Disclosed is an intrinsically gain flattened Erbium doped fiber amplifier comprising an inner core having relatively large A, an outer concentric depressed core having a smaller A, a cladding for covering the outer core wherein the hashed region e<r<d is doped with erbium ions.

Full Text	The present invention relates to an intrinsically gain flattened Erbium Doped fiber amplifier flattening the gain of an amplifier doped with erbium ion. More specifically the subject invention relates to: • a novel intrinsically gain-flattened three layers staircase profile c-band erbium doped silica based optical fiber amplifier (EDFA), having high optical gain and wide bandwidth; The invention is a highly efficient broad-band optical fiber amplifier which fully exploit the low loss band of silica fibers and greatly contributes to wavelength division multiplexed (WDM) technologies which contributed to dramatic change in the information carrying capacity (or transmissions capacity), to bear the explosively increased telecommunication traffic, of an optical fiber link. BACKGROUND OF THE INVENTION Various processes for gain flattening have been reported including A-l/P- codoped EDFA, fluoride or Telluride-based EDFAs and EDFAs with low population inversion operating at 1550 nm, as compared to silica host fluoride based EDFA yield better flat gain and bandwidth. A disadvantage of a single wavelength amplification at 1550 nm with silica based conventional EDFA is poor gain flatness over the amplification bond. This gain non-uniformity is determinate in WDM applications. In the case of multiple stage amplification, the useful spectral range for the system is drastically reduced. Correction of gain non-uniformity by equalisation requires internal or external filtering which increase the complexity of such system. Co-doping of germano-silica fibers with alumina broadens the gain spectrum but a large peak near 1530 nm remains. One solution consists in using flurozirconate glass hosts like ZBLAN instead of silica, but their splicing with silica based conventional transmission fiber results in significant coupling losses. The use of twin core fiber in EDFAs has also been suggested in many schemes of flattening the gain by using in different techniques wherein it is difficult to have simultaneous access of the two cores while in other systems a high noise figure is obtained. To overcome the drawbacks of the previous techniques and existing methods a novel intrinsically gain flattened - design has been developed in the subject invention of optical fibers by introducing the staircase fiber profile which is capable of providing flat gain over a wave-length span of approximately 34 nm. The subject design is fully compatible with the present conventional optical fiber link in terms of splicing and connection. The subject invention has a useful application in broad-board WDM optical communication system because it has high optical gain, wide-board width, better pump efficiency, flat gain, uniform amplification with reduced gain excursion and good tolerance of various parameters like core radii, refractive index etc. The subject invention is useful in the realization of gain flattened EDFAs without employing the number of extra external gain flattening components such as filters or use other mechanism like losing, thereby resulting in improved reliability of communication system. In the subject invention gain is less for wavelengths below 1540 nm as compared to a conventional EDFA while the gain is high for wavelengths greater than 1540 nm. This results in flattering of the gain spectrum. Changes in various parameters (case radii, refractive index, etc.) doesn't effect the phase matching condition hence gain spectrum. In the subject invention functional power increases from the inner core to the outer core with increase in wave length. This result in better pump efficiency. In the subject invention the noise figure is shown to decrease from about 6.5 dB at 1525 nm to 4.5 dm around 1560 mm. In the subject invention coupling efficiency is seen to be more than 95% of conventional undoped transmission fiber. The subject invention of staircase profile has three layers while cronial profile has four layers. Hence it is easier to fabricate a staircase fiber than the co-axial fiber. The subject invention is shown to exhibit uniform amplification characteristics with a gain excursion of ± 1 dB over a bond width of 34 mm with bg 980 mm pump wave length and a gain excursion of ± 1 dB over 30 mm in the c-bond operation regime with using 1480 mm pump wavelength. In the subject invention the process doped fiber of optical amplifier consist of two highly asymmetric cores separated by a depressed cladding with reflective erbium-doping in the outer core including the cladding. In the subject invention, the main advantage of new stair-care fiber structure over the coaxial fiber is the use of fiindamental mode at the pump wave-length as compared to the LP02 mode at 980 mm in case of the coaxial fiber, which requires an additional component (fiber brag grating) for LPOl to LP02 mode coupling. Accordingly the present invention relates to an intrinsically gain flattened Erbium Doped fiber an amplifier comprising an inner cone having relatively large A, an outer concentric depressed core having a smaller a cladding for covering the outer core wherein the bashed region e The subject invention may better be understood with reference to the accompanying drawings which are for illustrative purpose only and should not be constituted to limit the scope of the invention. BRIEF DESCRTPTTON OF THE ACCOMPANYING DRAWINGS Fig. 1 (a) shows the refractive indeed profiles (RIP) of two very highly dissimilar cores. Fig. 1(b) shows the RIP of the staircase design of gain flattened EDFA. Fig. 1(c) shows variation of the effective mode index with wave length for fibers having different core structures. Fig. 2 describes the normalised pump intensity distribution of the fundamental modes for staircase fiber profile across fiber cross-section. Fig. 3 describes the normalised intensity distribution of LPOl modes at signal wavelength from 1500 mm to 1600 mm). Fig. 4 shows comparison of gain spectra in single core coaxial and staircase EDFA. Fig. 5 shows figure spectra for staircase fiber and coaxial fiber. Fig. 6 shows comparison in fraction of the power in the doped region in the fundamental mode of signal of staircase EDF with the coaxial EDF. Fig 8-9 show pump power dependence of gain for signal core, coaxial and staircase EDF. Fig. 7, 8 show gain versus outsignal power for staircase power at pumping. Fig. 9 shows ASE power distribution (P&D) for. Fig. 10,11 show variation of gain spectrum of staircase EDF. PRINCIPAL OF OPERATION The principal behind gain flattening in such a design is as follows: In the proposed design of an intrinsically gain-flattened EDFA the flat gain is obtained by LPOl mode pumping at the pump wave length of 980 mm or 1480 mm. The proposed fiber consists of an inner core with a relatively large A which is immediately surrounded by an outer concentric depressed core of smaller A. followed by the cladding. The hacked region CLrLD is assumed to be doped with erbium ions (Fig. 1,2). The fiber parameters a, b, c, d Al and A2 [Al = (n12-n32)/n32, 1=1,2 where n, and n2 are the reflective indexes of inner and depressed core respectively and n, is the refractive index of cladding are so optimised that the effective index of the fundamental mode at 1540 mm of the staircase structure is close to the refractive index of the depressed core, i.e., n2. Hence for wavelengths below 1540 mm. The fundamental mode will have an effective index greater than Uj and hence will have major fraction of power within the inner core, i.e. up to r=a, whereas for wave lengths above 1540 mm, the refractive index will be below Uj and thus at these wavelengths the minority of power will extend up to the outer core radius, i.e., r=b. Thus as wavelength changes from below to above 1540 mm, the mode filed profile would undergo significant changes. When the outer core and part of the cladding are reflectively doped with erbium ions, because of a higher overlap of signal with the doped region for wavelengths greater than 1540 mm and lower overlap for wavelengths less than 1540, the gain spectrum gets modified. The gain for wavelengths less than 1540 mm will be less as compared to a conventional EDFA. While the gain for wavelengths greater than 1540 mm will be higher. Thus this is again expected to yield flattening of the gain spectrum. In order to obtain suitable parameters of the fiber designs the following procedure is adopted : 1. An inner core is chosen with a high ( 2%) and small core radius (2 µm). With these parameters the fiber consisting of only the first core is single moded in the wavelength region 1520-1560 nm. 2. Calculating the effective index and mode filed profile of the LP01 mode of the fiber. 3. To have high asymmetry in the refractive index between the two cores, then choose a relatively small 2 (~ 0.7%) for the outer core. 4. For a particular inter-core separation (typically 3.a for the coaxial profile and zero for staircase profile), we obtained by numerical iteration that particular core radius of a fiber consisting of the second core alone for which the effective index of the fundamental mode matches that of the individual first core fiber at 1540 nm. 5. We then obtain the effective indices of the fundamental modes of the concentric core coaxial and staircase fibers as a function of wavelenght. Since the presence of the second core would perturb the effective index of the outer core in either designs, the inner and outer cores may not necessarily be exactly phase matched at the chosen wavelenght. 6. In order to take into account the effect of perturbation in both designs, the radius of the outer core is changed to alter the phase matched wavelenght to coincide with the desired operating wavelength i.e. 1540 nm. 7. Now these coaxial and staircase fibers are assumed to be uniformly doped with erium inos in the regions specified in Fig 1 and the gain spectra are evaluated using the numerical simulation. The various parameters of the doped coaxial and staircase fibers are changed separately in nested loop styles to obtain the maximum flatness in their gain-spectra. This is done keeping the inner core radius constant. The fiber parameters for coaxial and staircase profiles (at 1550 mm) corresponding to the max. flattened gain spectrum for 980 and 1480 mm pump wave lengths are tabulated in table 1; the variation of refractive index with the wavelength is calculated by using sellemeir's eqn [Adoms (1981)]. (TABLE REMOVED) Fiber parameters for single core, co-axial and staircase EDFs (for 980 mm and 1480 mm) corresponding to the maximum flattened gain spectrum at 1550 mm wavelength. The gain versus pump power characteristics are the first and the most important measure of EDFA performance. The other important characteristics are gain versus signal power curves, the amplified spontaneous emission (ASE) noise and optical noise figure (NF). The core redii and relative refractive indexes differences play an important role in phase matching. Any variation in these parameters does not affect the phase matching condition and hence the gain spectrum. We claim: 1. An intrinsically gain flattened Erbium doped fiber amplifier comprising an inner core having relatively large relative refractive index difference 1 up to 0.02 and small core radius up to 2um, an outer concentric depressed core having a smaller relative refractive index difference 2, up to 0.007, and a cladding for covering the outer core, characterized in that a portion between the outer depressed core and cladding is doped with erbium ions to form a staircase profile, wherein the inner core is single moded in the wavelength region 1520-1550 nm, and wherein separation between inner core and outer core is zero, and wherein the staircase profile fiber amplifier uses fundamental mode at pump wavelength, and wherein refractive index of the inner core, the depressed core and the cladding are so optimized that effecting index of fundamental mode of the staircase profile fiber amplifier at 1540 nm is close to refractive index of the depressed core.

Full Text

The present invention relates to an intrinsically gain flattened Erbium Doped fiber amplifier flattening the gain of an amplifier doped with erbium ion.
More specifically the subject invention relates to:
• a novel intrinsically gain-flattened three layers staircase profile c-band erbium doped silica based optical fiber amplifier (EDFA), having high optical gain and wide bandwidth;
The invention is a highly efficient broad-band optical fiber amplifier which fully exploit the low loss band of silica fibers and greatly contributes to wavelength division multiplexed (WDM) technologies which contributed to dramatic change in the information carrying capacity (or transmissions capacity), to bear the explosively increased telecommunication traffic, of an optical fiber link.
BACKGROUND OF THE INVENTION
Various processes for gain flattening have been reported including A-l/P- codoped EDFA, fluoride or Telluride-based EDFAs and EDFAs with low population inversion operating at 1550 nm, as compared to silica host fluoride based EDFA yield better flat gain and bandwidth.
A disadvantage of a single wavelength amplification at 1550 nm with silica based conventional EDFA is poor gain flatness over the amplification bond. This gain non-uniformity is determinate in WDM applications.
In the case of multiple stage amplification, the useful spectral range for the system is drastically reduced. Correction of gain non-uniformity by equalisation requires internal or external filtering which increase the complexity of such system.
Co-doping of germano-silica fibers with alumina broadens the gain spectrum but a large peak near 1530 nm remains.
One solution consists in using flurozirconate glass hosts like ZBLAN instead of silica, but their

splicing with silica based conventional transmission fiber results in significant coupling losses.
The use of twin core fiber in EDFAs has also been suggested in many schemes of flattening the gain by using in different techniques wherein it is difficult to have simultaneous access of the two cores while in other systems a high noise figure is obtained.
To overcome the drawbacks of the previous techniques and existing methods a novel intrinsically gain flattened - design has been developed in the subject invention of optical fibers by introducing the staircase fiber profile which is capable of providing flat gain over a wave-length span of approximately 34 nm. The subject design is fully compatible with the present conventional optical fiber link in terms of splicing and connection.
The subject invention has a useful application in broad-board WDM optical communication system because it has high optical gain, wide-board width, better pump efficiency, flat gain, uniform amplification with reduced gain excursion and good tolerance of various parameters like core radii, refractive index etc.
The subject invention is useful in the realization of gain flattened EDFAs without employing the number of extra external gain flattening components such as filters or use other mechanism like losing, thereby resulting in improved reliability of communication system.
In the subject invention gain is less for wavelengths below 1540 nm as compared to a conventional EDFA while the gain is high for wavelengths greater than 1540 nm. This results in flattering of the gain spectrum.
Changes in various parameters (case radii, refractive index, etc.) doesn't effect the phase matching condition hence gain spectrum.
In the subject invention functional power increases from the inner core to the outer core with increase in wave length. This result in better pump efficiency.
In the subject invention the noise figure is shown to decrease from about 6.5 dB at 1525 nm
to 4.5 dm around 1560 mm.
In the subject invention coupling efficiency is seen to be more than 95% of conventional undoped transmission fiber.
The subject invention of staircase profile has three layers while cronial profile has four layers. Hence it is easier to fabricate a staircase fiber than the co-axial fiber.
The subject invention is shown to exhibit uniform amplification characteristics with a gain excursion of ± 1 dB over a bond width of 34 mm with bg 980 mm pump wave length and a gain excursion of ± 1 dB over 30 mm in the c-bond operation regime with using 1480 mm pump wavelength.
In the subject invention the process doped fiber of optical amplifier consist of two highly asymmetric cores separated by a depressed cladding with reflective erbium-doping in the outer core including the cladding.
In the subject invention, the main advantage of new stair-care fiber structure over the coaxial fiber is the use of fiindamental mode at the pump wave-length as compared to the LP02 mode at 980 mm in case of the coaxial fiber, which requires an additional component (fiber brag grating) for LPOl to LP02 mode coupling.
Accordingly the present invention relates to an intrinsically gain flattened Erbium Doped fiber an amplifier comprising an inner cone having relatively large A, an outer
concentric depressed core having a smaller a cladding for covering the outer core wherein the bashed region e The subject invention may better be understood with reference to the accompanying drawings which are for illustrative purpose only and should not be constituted to limit the scope of the invention.
BRIEF DESCRTPTTON OF THE ACCOMPANYING DRAWINGS
Fig. 1 (a) shows the refractive indeed profiles (RIP) of two very highly dissimilar cores.
Fig. 1(b) shows the RIP of the staircase design of gain flattened EDFA.
Fig. 1(c) shows variation of the effective mode index with wave length for fibers having different core structures.
Fig. 2 describes the normalised pump intensity distribution of the fundamental modes for staircase fiber profile across fiber cross-section.
Fig. 3 describes the normalised intensity distribution of LPOl modes at signal wavelength from 1500 mm to 1600 mm).
Fig. 4 shows comparison of gain spectra in single core coaxial and staircase EDFA.
Fig. 5 shows figure spectra for staircase fiber and coaxial fiber.
Fig. 6 shows comparison in fraction of the power in the doped region in the fundamental mode of signal of staircase EDF with the coaxial EDF.
Fig 8-9 show pump power dependence of gain for signal core, coaxial and staircase EDF.

Fig. 7, 8 show gain versus outsignal power for staircase power at pumping.
Fig. 9 shows ASE power distribution (P&D) for.
Fig. 10,11 show variation of gain spectrum of staircase EDF.
PRINCIPAL OF OPERATION
The principal behind gain flattening in such a design is as follows:
In the proposed design of an intrinsically gain-flattened EDFA the flat gain is obtained by LPOl mode pumping at the pump wave length of 980 mm or 1480 mm. The proposed fiber consists of an inner core with a relatively large A which is immediately surrounded by an outer concentric depressed core of smaller A. followed by the cladding. The hacked region CLrLD is assumed to be doped with erbium ions (Fig. 1,2). The fiber parameters a, b, c, d Al and A2 [Al = (n12-n32)/n32, 1=1,2 where n, and n2 are the reflective indexes of inner and depressed core respectively and n, is the refractive index of cladding are so optimised that the effective index of the fundamental mode at 1540 mm of the staircase structure is close to the refractive index of the depressed core, i.e., n2. Hence for wavelengths below 1540 mm. The fundamental mode will have an effective index greater than Uj and hence will have major fraction of power within the inner core, i.e. up to r=a, whereas for wave lengths above 1540 mm, the refractive index will be below Uj and thus at these wavelengths the minority of power will extend up to the outer core radius, i.e., r=b. Thus as wavelength changes from below to above 1540 mm, the mode filed profile would undergo significant changes. When the outer core and part of the cladding are reflectively doped with erbium ions, because of a higher overlap of signal with the doped region for wavelengths greater than 1540 mm and lower overlap for wavelengths less than 1540, the gain spectrum gets modified. The gain for wavelengths less than 1540 mm will be less as compared to a conventional EDFA.
While the gain for wavelengths greater than 1540 mm will be higher. Thus this is again expected to yield flattening of the gain spectrum.
In order to obtain suitable parameters of the fiber designs the following procedure is adopted :
1. An inner core is chosen with a high ( 2%) and small core radius (2 µm). With these parameters the fiber consisting of only the first core is single moded in the wavelength region 1520-1560 nm.
2. Calculating the effective index and mode filed profile of the LP01 mode of the fiber.
3. To have high asymmetry in the refractive index between the two cores, then choose a relatively small 2 (~ 0.7%) for the outer core.
4. For a particular inter-core separation (typically 3.a for the coaxial profile and zero for staircase profile), we obtained by numerical iteration that particular core radius of a fiber consisting of the second core alone for which the effective index of the fundamental mode matches that of the individual first core fiber at 1540 nm.
5. We then obtain the effective indices of the fundamental modes of the concentric core coaxial and staircase fibers as a function of wavelenght. Since the presence of the second core would perturb the effective index of the outer core in either designs, the inner and outer cores may not necessarily be exactly phase matched at the chosen wavelenght.
6. In order to take into account the effect of perturbation in both designs, the radius of the outer core is changed to alter the phase matched wavelenght to coincide with the desired operating wavelength i.e. 1540 nm.
7. Now these coaxial and staircase fibers are assumed to be uniformly doped with erium inos in the regions specified in Fig 1 and the gain spectra are evaluated using
the numerical simulation. The various parameters of the doped coaxial and staircase fibers are changed separately in nested loop styles to obtain the maximum flatness in their gain-spectra. This is done keeping the inner core radius constant.
The fiber parameters for coaxial and staircase profiles (at 1550 mm) corresponding to the max. flattened gain spectrum for 980 and 1480 mm pump wave lengths are tabulated in table 1; the variation of refractive index with the wavelength is calculated by using sellemeir's eqn [Adoms (1981)].
(TABLE REMOVED)

Fiber parameters for single core, co-axial and staircase EDFs (for 980 mm and 1480 mm) corresponding to the maximum flattened gain spectrum at 1550 mm wavelength.
The gain versus pump power characteristics are the first and the most important measure of EDFA performance. The other important characteristics are gain versus signal power curves, the amplified spontaneous emission (ASE) noise and optical noise figure (NF). The core redii and relative refractive indexes differences play an important role in phase matching. Any variation in these parameters does not affect the phase matching condition and hence the gain spectrum.

We claim:
1. An intrinsically gain flattened Erbium doped fiber amplifier comprising an inner core having relatively large relative refractive index difference 1 up to 0.02 and small core radius up to 2um, an outer concentric depressed core having a smaller relative refractive index difference 2, up to 0.007, and a cladding for covering the outer core, characterized in that a portion between the outer depressed core and cladding is doped with erbium ions to form a staircase profile,
wherein the inner core is single moded in the wavelength region 1520-1550 nm, and wherein separation between inner core and outer core is zero, and
wherein the staircase profile fiber amplifier uses fundamental mode at pump wavelength, and wherein refractive index of the inner core, the depressed core and the cladding are so optimized that effecting index of fundamental mode of the staircase profile fiber amplifier at 1540 nm is close to refractive index of the depressed core.

Documents:

924-DEL-2002-Abstract-(25-08-2011).pdf

924-del-2002-abstract.pdf

924-DEL-2002-Claims-(15-02-2012).pdf

924-DEL-2002-Claims-(25-08-2011).pdf

924-del-2002-claims.pdf

924-DEL-2002-Correspondence Others-(15-02-2012)..pdf

924-DEL-2002-Correspondence Others-(15-02-2012).pdf

924-DEL-2002-Correspondence Others-(25-08-2011).pdf

924-del-2002-correspondence-others.pdf

924-del-2002-correspondence-po.pdf

924-DEL-2002-Description (Complete)-(15-02-2012).pdf

924-del-2002-description (complete).pdf

924-del-2002-description (provisinal).pdf

924-DEL-2002-Drawings-(25-08-2011).pdf

924-del-2002-drawings.pdf

924-del-2002-form-1.pdf

924-DEL-2002-Form-13-(15-02-2012).pdf

924-del-2002-form-18.pdf

924-DEL-2002-Form-2-(15-02-2012).pdf

924-DEL-2002-Form-2-(25-08-2011).pdf

924-del-2002-form-2.pdf

924-del-2002-form-26.pdf

924-del-2002-form-3.pdf

924-del-2002-form-4.pdf

924-del-2002-form-5.pdf

924-DEL-2012-Correspondence Others-(06-03-2012).pdf

924-DEL-2012-GPA-(06-03-2012).pdf

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Patent Number

259783

Indian Patent Application Number

924/DEL/2002

PG Journal Number

13/2014

Publication Date

28-Mar-2014

Grant Date

27-Mar-2014

Date of Filing

12-Sep-2002

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY

Applicant Address

HAUZ KHAS, NEW DELHI-110 016 INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	KRISHNA THYAGARAJAN	DEPARTMENT OF PHYSICS, INDIAN INSTITUTE OF TECHNOLOGY, HAUZ KHAS, NEW DELHI-110 016 INDIA
2	JAGNEET KAUR ANAND	DEPARTMENT OF PHYSICS, KESHAV MAHAVIDHYALA, KESHAV PURAM, UNIVERSITY OF DELHI, NEW DELHI-110 035, INDIA

PCT International Classification Number

G02B6/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA