CLIAMS:What is claimed is: 1. A silica optical fiber, said optical fiber comprising: i. a core, said core further comprising: a first region (102), said first region (102) being cylindrical in shape, said first region (102)lying symmetrically around a longitudinal axis of the optical fiber, said first region (102) extending along said longitudinal axis, and said first region (102) having a radius r1; a second region (104), said second region (104) lying symmetrically around said first region (102), said second region(104) extending along said longitudinal axis, and said second region (104) lying between said radial distance r1anda radial distance r2around said longitudinal axis, wherein said r2 is greater than said r1; a third region (106), said third region (106) lying symmetrically around said second region (104), said third region (106) extending along said longitudinal axis, and said third region (106) lying between said radial distance r2 and a radial distance r3 around said longitudinal axis, wherein said r3 is greater than said r2; ii. a cladding, said cladding surrounding said core, and refractive index at any location within said cladding is lower than refractive index at any location within said core, said cladding further comprising: a fourth region (108), said fourth region (108) lying symmetrically around said third region (106), said fourth region (108) extending along said longitudinal axis, and said fourth region (108) lying between said radial distance r3anda radial distance r4 around said longitudinal axis, wherein said r4 is greater than said r3; and a fifth region (110) symmetrically surrounding said fourth region (108), said fifth region (110) extending along said longitudinal axis, and said fifth region (110) lying between said radial distance r4 and a radial distance r5 around said longitudinal axis, wherein said r5is greater than said r4; wherein said first region (102), said second region (104) and said fourth region (108) are doped with at least one rare-earth element based dopant, said at least one rare-earth element based dopant providing rare-earth element ions; and said third region (106) and said fifth region (110) are not doped with any rare-earth element based dopant; wherein said first region (102), said second region (104) and said fourth region (108) are doped with at least one rare-earth element based dopant in a manner such that: within said first region (102),concentration of rare-earth element ions increases with increase in radial distance from said longitudinal axis; within said second region (104), concentration of rare-earth element ions remains constant with an increase in radial distance from said longitudinal axis; within said fourth region (108), concentration of rare-earth element ions decreases with increase in radial distance from said longitudinal axis; concentration of rare-earth element ions at any location within said second region is greater than or equal to concentration of rare-earth element ions at any location within said first region; and concentration of rare-earth element ions at any location within said second region is greater than concentration of rare-earth element ions at any location within said fourth region. 2. The optical fiber as claimed in claim 1, wherein said first region (102) and said second region (104) are also doped with GeO2. 3. The optical fiber as claimed in claim 1, wherein said third region (106) is doped with GeO2. 4. The optical fiber as claimed in claim 1, wherein said at least one rare-earth element based dopant is based one of the following rare-earth elements: i. Erbium ii. Ytterbium iii. Thulium iv. Neodymium, and v. Holmium. 5. The optical fiber as claimed in claim 1, wherein said rare-earth element ions include ions of one or more of following rare-earth elements: vi. Erbium vii. Ytterbium viii. Thulium ix. Neodymium, and x. Holmium. 6. The optical fiber as claimed in claim 1, wherein said at least one rare-earth element based dopant is Er2O3. 7. The optical fiber as claimed in claim 1, wherein r4< r5= (2 × r4). 8. The optical fiber as claimed in claim 1, wherein r4= (2 × r3). 9. The optical fiber as claimed in claim 1, wherein 1 µm = r3= 4.5 µm. Dated: 4th Day of May, 2015 Signature Arun Kishore Narasani Patent Agent ,TagSPECI: TECHNICAL FIELD [0001] The present invention relates to the field of optical fibers that are doped with dopants based on rare-earth elements, and, in particular, relates to rare-earth element doped optical fiber amplifiers. BACKGROUND [0002] Optical fiber cables are the backbone of modern communication infrastructure and systems. Since optical fibers are practically immune to electrical interference and facilitate availability of much larger bandwidths over conventionally used copper wires, optical fiber cables are rapidly replacing conventionally used copper wire cables for communication. Our reliance on optical fibers for telecommunications, broadband communication, communication over passive optical networks, and the like is growing day by day. With advancement in technology, apart from communications, optical fibers are increasingly being widely used for sensor applications, medicinal surgical applications, visible light transmission and many such fields. [0003] In general, a strand of optical fiber includes a core and a cladding. The core is a cylindrical region lying at the center of the optical fiber and has a refractive index n1. Further, the core surrounds a longitudinal axis of the optical fiber and also lies symmetrically around it. The cladding, having a refractive index n2, n2 being less than n1, symmetrically surrounds the core. Optical signals to be transmitted through the optical fiber are fired in to the core of the optical fiber. Transmitted optical signals travel through the core and, while travelling, are kept confined in the core in accordance with a well-known scientific phenomenon of total internal reflection. As the transmitted signals travel through the core, they suffer attenuation along the path of travel. Reasons for attenuation while travelling are well known and are excluded from further discussion herein. After travelling a certain distance, the optical signals become so weak that they need a mandatory amplification for travelling any further. If faithful amplification of attenuated signals is not done at appropriate location, they would eventually be lost or at least get attenuated to a level from where their re-generation would be almost impossible. [0004] One of the popular methods for amplifying attenuated optical signals and to restore them to their original levels is to use an optical fiber amplifier at regular interval/s in an optical fiber route connecting a transmitting station to a destination (or a receiving station). Optical signals which get attenuated after travelling a specific length of optical fiber are fed in to an optical fiber amplifier. Within the optical fiber amplifier, such signals are amplified to desired levels. Thereafter, amplified optical signals are ejected from the output of the optical signal amplifier and are fed to the optical fiber route for transmission towards destination. [0005] Rare-earth Doped Optical Fibers (referred as ‘REDOF’ throughout the text hereinafter) are well known and have conventionally been used as amplifiers for optical signals. An REDOF is a type of optical fiber. Basically, the core of an REDOF is doped with rare-earth element ions (such as Er3+, provided by doping core with Er2O3). When doped rare-earth element ions are pumped through a pumping laser, amplification of optical signals travelling through the REDOF is achieved. The process of amplification of optical signals is well known in the art and is not explained in details herein for obvious reasons. In summary, with support of a pumping laser, REDOFs are capable of amplifying optical signals travelling through them. An optical signal fed in to one end of an REDOF would gradually get amplified while travelling to the other end of the REDOF. In a lengthy optical fiber communication route, when becomes necessary to ensure that optical signals travelling through it do not get lost due to over attenuation, one or more REDOFs are included at appropriate locations. While travelling from the transmitting end towards a destination (or receiving end) of an optical fiber route, attenuated optical signals enter an REDOF installed at a particular location of the route. Attenuated optical signals enter the REDOF from one end, get amplified while travelling through it. Post amplification, optical signals exit the REDOF from other end, and are fed in to the optical fiber route for travelling towards the destination. [0006] In spite of their usefulness, currently known REDOFs suffer with a major drawback. Apart from amplifying, currently known REDOFs are prone to induce large background attenuation on optical signals travelling through it, i.e. in addition to amplification, an optical signal also suffers with large attenuation while travelling through the REDOF. As a result of this drawback, amplification efficiency of a given length of REDOF gets reduced. In other words, in spite of having a capability to amplify travelling optical signals to larger levels, only a fractional amount of signal amplification is achieved in the given length of the REDOF. To overcome this drawback, larger lengths of REDOFs and high pumping power are used to achieve an acceptable/desired level of optical signal amplification cumulatively. However, requirement of larger lengths of REDOF and high pumping power comes with additional costs and support resources. [0007] Hence, there is an acute need for an REDOF which would: i. Achieve efficient amplification of optical signals travelling through it, ii. Induce lesser attenuation on optical signals travelling through it, and iii. Achieve desired levels of amplification of optical signals, travelling through the REDOF, in relatively smaller lengths in comparison to conventionally known REDOFs. OBJECT OF THE INVENTION [0008] An object of the present invention is to provide a REDOF which would achieve efficient amplification of optical signals travelling through it. [0009] Another object of the present invention is to provide an REDOF which would induce lesser attenuation of optical signals travelling through it. [0010] Another object of the present invention is to provide an REDOF which would achieve desired levels of amplification of optical signals, travelling through the REDOF, in relatively smaller lengths in comparison to conventionally known REDOFs. [0011] Another object of the present invention is to provide a profile of concentration of rare-earth element ions for an REDOF that would facilitate reduction in attenuation of optical signals travelling through it. [0012] Another object of the present invention is to provide a profile of concentration of rare-earth element ions for an REDOF that would facilitate efficient amplification of optical signals travelling through it. [0013] Another object of the present invention is to provide a profile of concentration of rare-earth element ions for an REDOF that would facilitate desired amplification of optical signals travelling through the REDOF, in relatively smaller lengths in comparison to conventionally known REDOFs SUMMARY [0014] Accordingly, the invention provides silica (glass based) REDOF which overcomes the drawbacks of conventional REDOFs as described above. An REDOF, in accordance with the present invention, includes a core and a cladding. Refractive index at any location within the core is greater than refractive index at any location within the cladding. The core is cylindrical and lies symmetrically around a longitudinal axis of the REDOF. The longitudinal axis of the REDOF passes through the core. The cladding symmetrically surrounds the core and lies symmetrically around the longitudinal axis of the REDOF. The core further includes a first region, a second region, and a third region. All three regions included in the core i.e. the first region, the second region and the third region lie symmetrically around the longitudinal axis of the REDOF. The longitudinal axis of the REDOF passes through the first region. The second region surrounds the first region, and the third region surrounds the second region. Within a cross-section of the REDOF, said cross-section being taken along a plane perpendicular to the longitudinal axis of the REDOF, the center of the cross-section (which is a point from where the longitudinal axis would pass through the REDOF) is included in first region. Further, within the cross-section of the REDOF, the second region and the third region lie concentrically around the first region. Within the cross-section of the REDOF, the first region extends to a radial distance r1from the center of cross-section. Within the cross-section of the REDOF, the second region lies between a radial distance r1 and a radial distance r2 from the center of cross-section (r2 being greater than r1). Still further, within the cross-section of the REDOF, the third region lies between a radial distance r2 and radial distance r3 from the center of cross-section(r3 being greater than r2, and r3 lies in a range of about 1-4.5 microns). [0015] The cladding of the REDOF further includes two regions namely, a fourth region and a fifth region. While the fourth region surrounds the core of the REDOF, the fifth region surrounds the fourth region. Both, the fourth region and the fifth region lie symmetrically around the longitudinal axis of the REDOF. And, within the cross-section of the REDOF, said cross-section being taken along a plane perpendicular to the longitudinal axis of the REDOF, the fourth region and the fifth region lie concentrically around the third region of the REDOF. Within the cross-section of the REDOF, the fourth region lies between a radial distance r3 and a radial distance r4 from the center of cross-section (r4 being greater than r3). Still further, within the cross-section of the REDOF, the fifth region lies between a radial distance r4 and a radial distance r5 from the center of cross-section (r5 being greater than r4). [0016] The first region, the second region and the fourth region are doped with one or more rare-earth element based dopants for providing rare-earth element ions (for example, the first region, the second region and the fourth region are doped with Er2O3 for providing Erbium ions (Er3+)).Doping of the first region, the second region and the fourth region with one or more rare-earth element based dopants is done in a manner such that within the first region, concentration of rare-earth element ions increases from the longitudinal axis of the REDOF towards radial distance r1from the longitudinal axis. Within the second region, concentration of rare-earth element ions remains constant. And, within the fourth region, concentration of rare-earth element ions decreases from radial distance r3from the longitudinal axis towards the radial distancer4from the longitudinal axis. Concentration of the rare-earth element ions in the second region is greater than or equal to concentration of rare-earth element ions at any location within the first region. Concentration of rare-earth element ions in the second region is greater than the concentration of rare-earth element ions at any location in the fourth region. The third region and the fifth region are not doped with any rare-earth element based dopants. [0017] In an embodiment of the REDOF, in accordance with the present invention: i. the first region and the second region are doped with Er2O3 (for providing Erbiumions, Er3+) and Germanium Oxide (GeO2) ii. the third region is doped only with GeO2(a non rare-earth element based dopant), iii. the fourth region is doped only withEr2O3 (for providing Erbium ions, Er3+), iv. the fifth region is kept free from any dopants (i.e. its neither doped with GeO2nor with Er2O3), and v. r4 = (2×r3) STATEMENT OF THE INVENTION [0018] The invention provides silica (glass based) REDOF. An REDOF, in accordance with the present invention, includes a core and a cladding. Refractive index at any location within the core is greater than refractive index at any location within the cladding. The core is cylindrical and lies symmetrically around a longitudinal axis of the REDOF. The longitudinal axis of the REDOF passes through the core. The cladding symmetrically surrounds the core and lies symmetrically around the longitudinal axis of the REDOF. The core further includes a first region, a second region, and a third region. All three regions included in the core i.e. the first region, the second region and the third region lie symmetrically around the longitudinal axis of the REDOF. The longitudinal axis of the REDOF passes through the first region. The second region surrounds the first region, and the third region surrounds the second region. Within a cross-section of the REDOF, said cross-section being taken along a plane perpendicular to the longitudinal axis of the REDOF, the center of the cross-section (which is a point from where the longitudinal axis would pass through the REDOF) is included in first region. Further, within the cross-section of the REDOF, the second region and the third region lie concentrically around the first region. Within the cross-section of the REDOF, the first region extends to a radial distance r1 from the center 101 of cross-section. Within the cross-section of the REDOF, the second region lies between a radial distance r1 and a radial distance r2 from the center 101 of cross-section (r2 being greater than r1). Still further, within the cross-section of the REDOF, the third region lies between a radial distance r2 and radial distance r3 from the center 101 of cross-section (r3 being greater than r2, and r3 lying in the range of 1- 4.5 microns). [0019] The cladding of the REDOF further includes two regions namely, a fourth region and a fifth region. While the fourth region surrounds the core of the REDOF, the fifth region surrounds the fourth region. Both, the fourth region and the fifth region lie symmetrically around the longitudinal axis of the REDOF. And, within the cross-section of the REDOF, said cross-section being taken along a plane perpendicular to the longitudinal axis of the REDOF, the fourth region and the fifth region lie concentrically around the third region of the REDOF. Within the cross-section of the REDOF, the fourth region lies between a radial distance r3 and a radial distance r4 from the center 101 of cross-section (r4 being greater than r3). Still further, within the cross-section of the REDOF, the fifth region lies between a radial distance r4 and a radial distance r5 from the center 101 of cross-sec (r5 being greater than r4). [0020] The first region, the second region and the fourth region are doped with one or more rare-earth element based dopants for providing rare-earth element ions (for example, the first region, the second region and the fourth region are doped with Er2O3 for providing Erbium ions (Er3+)). Doping of the first region, the second region and the fourth region with one or more rare-earth element based dopants is done in a manner such that within the first region, concentration of rare-earth element ions increases from the longitudinal axis of the REDOF towards radial distance r1from the longitudinal axis. Within the second region, concentration of rare-earth element ions remains constant. And, within the fourth region, concentration of rare-earth element ions decreases from radial distance r3 from the longitudinal axis towards the radial distance r4from the longitudinal axis. Concentration of the rare-earth element ions in the second region is greater than or equal to concentration of rare-earth element ions at any location within the first region. Concentration of rare-earth element ions in the second region is greater than the concentration of rare-earth element ions at any location in the fourth region. The third region and the fifth region are not doped with any rare-earth element based dopants. BRIEF DESCRIPTION OF FIGURES [0021] Having thus described the invention in general terms, reference will now be made to the accompanying figures, where: [0022] FIG. 1 illustrates a cross-sectional view of an REDOF in accordance with a first embodiment of the present invention. [0023] FIG. 2 illustrates a concentration profile of rare-earth element ions of the REDOF in accordance with the first embodiment of the present invention. [0024] FIG. 3 is a graph illustrating plots of optical gain achieved in two sample embodiments of REDOF prepared in accordance with the present invention. FIG. 3 also illustrates plot of optical gain obtained in REDOF which was not prepared in accordance with the present invention; [0025] FIG. 4 is a graph illustrating plots of optical gain achieved in three sample embodiments of REDOF, prepared in accordance with the present invention, based on slope of concentration profile of rare-earth element ions in their respective first region. [0026] FIG. 5 is a graph illustrating plots of optical gain achieved in two sample embodiments of REDOF, prepared in accordance with the present invention, based on slope of concentration profile of rare-earth element ions in the fourth region. [0027] FIG. 6 is a graph illustrating plots of optical gain achieved in three sample embodiments of REDOF, prepared in accordance with the present invention, based on their respective core radius. [0028] It should be noted that the accompanying figures are intended to present illustrations of exemplary embodiments of the present invention. These figures are not intended to limit the scope of the present invention. It should also be noted that accompanying figures are provided for a clear understanding of the invention and are not necessarily drawn to scale. DETAILED DESCRIPTION [0029] Reference will now be made in detail to an embodiment of the present invention in conjunction with accompanying figures. It is to be noted that the present invention relates to silica REDOFs. Silica REDOFs as provided by the present invention are made of glass which may be doped with suitable dopants as desired. Unless stated otherwise, for all the text which follows herein below, the term REDOF would represent ‘Silica REDOF’. The embodiments described herein are not intended to limit the scope of the invention, and the present invention should not be construed as limited to the embodiment described. This invention maybe embodied in various other forms which would be obvious to a person skilled in the art. All embodiments which are in accordance with the present invention (including the ones arising by incorporating obvious modifications over them), are fully covered within the scope of the present invention and its claims. It should be understood that the accompanying figures are intended to provide a better description and understanding of an embodiment of the invention described below and are not necessarily drawn to scale. In the drawings, like numbers refer to like elements throughout, and thicknesses and dimensions of some components may be exaggerated for providing better clarity and ease of understanding. [0030] It should be noted that the terms "first", "second", and the like, herein do not denote any order, ranking, quantity, or importance, but rather are used to distinguish one element from another. Further, the terms "a" and "an" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. [0031] FIG. 1 illustrates a cross-sectional view of aREDOF100 in accordance with a first embodiment of the present invention. The cross-sectional view of REDOF100 as illustrated in FIG. 1 is taken along a plane perpendicular to a longitudinal axis of the REDOF100. Since FIG. 1 illustrates a cross-sectional view of the REDOF 100, the longitudinal axis of the fiber is not shown. However, it is obvious that the longitudinal axis of REDOF 100would pass through the center101 of cross-section of REDOF 100 illustrated in FIG. 1. [0032] The REDOF 100 includes a core and a cladding. Any location within the core of the REDOF 100 has a higher refractive index than any location in the cladding of the REDOF 100. While the core of the REDOF100 provides a path for optical signals to travel through the REDOF 100, the cladding (having a lower refractive index than the core), in accordance with well-known scientific principle of total internal reflection, ensures that travelling optical signals are confined to the core and do not escape from it. As shown in FIG. 1, the REDOF100includes a first region 102, a second region 104 and a third region 106. All three regions i.e. the first region 102, the second region 104 and the third region 106 collectively form the core of the REDOF 100. The second region 104 surrounds the first region102 and the third region 106 surrounds the second region 104. The first region 102is cylindrical and lies symmetrically around a longitudinal axis (not illustrated in FIG. 1) of the REDOF100. The longitudinal axis of the REDOF100 passes through the first region 102. Both the second region 104 and the third region 106 also lie symmetrically around the longitudinal axis of the REDOF 100. In the cross-section of FIG. 1, the longitudinal axis of the fiber would pass through the center101 of the cross-section. It is to be noted that center 101 of the cross-section of REDOF100 illustrated in FIG. 1 is included in the first region 102.The second region 104 and the third region 106concentrically surround the first region 102.Also illustrated in FIG.1 are a fourth region 108and a fifth region 110. Both, the fourth region 108and the fifth region 110collectively form the cladding of the REDOF100. And, both, the fourth region 108and the fifth region 110concentrically surround the third region. Further, in the illustration of FIG.1, the fourth region 108and the fifth region 110also lie symmetrically around the center101 of the cross-section of the REDOF100. It is to be noted that refractive index at any location within the core of REDOF100 is greater than refractive index at any location within the cladding of the REDOF100. [0033] The REDOF100 is made of silica or glass, and various portions of REDOF100 are either doped or left undoped in accordance with the present invention. The first region 102and the second region 104are doped with an oxide of Erbium, Er2O3. Erbium is a well-known rare-earth element and Er2O3provides Erbium ions (Er3+). The third region 106 is not doped with any rare-earth element based dopant. The fourth region 108 is also doped with Er2O3. In the fourth region 108 too, Er2O3provides Erbium ions (Er3+).The fifth region 110is not doped with any rare-earth element based dopant. Further, first region 102,the second region 104 and the third region 106 are also doped with GeO2(a non rare-earth element based dopant).It is to be noted that doping of the first region 102,the second region 104 and the third region 106 with GeO2 is necessary to provide greater refractive index to the core in comparison to the cladding. The fifth region 110 is made of pure SiO2 and is left undoped. [0034] Next, FIG. 2 illustrates a concentration profile of rare-earth element ion of REDOF 100(i.e. Er3+ in the present embodiment). In FIG. 2, concentration of Er3+in various regions of REDOF100 is plotted against radial distances of respective regions from the center101 of cross-section of the REDOF100as illustrated in FIG. 1. Concentration of Er3+is provided along the vertical axis and is expressed in units of number of ions/meter3. Similarly, radial distances of respective regions of the REDOF100 from the center 101 of cross-section of the REDOF100 are provided along the horizontal axis. Radial distances on the horizontal axis are expressed in micrometers (µm).As shown in FIG. 2, the first region 102extendsto a radial distance r1from the center 101 of cross-section of REDOF100.The second region 104lies between radial distance r1and radial distance r2 from the center 101 of cross-section of REDOF 100.The third region 106lies between radial distance r2and radial distance r3 from the center 101 of cross-section of REDOF 100.The fourth region 108lies between radial distancer3and radial distance r4 from the center 101 of cross-section of REDOF 100. The fifth region 110 lies between radial distance r4 and radial distance r5 from the center 101 of cross-section of REDOF 100. [0035] As illustrated in FIG. 2, the concentration of Er3+ionsin the center 101 of cross-section of REDOF 100 is C1.While moving towards the outermost radius r5 of the REDOF 100, the concentration of Er3+ ions at the beginning of the second region 104 (immediately after the interface of the first region 102 and the second region 104) is C2.Since the second region 104 has a substantially uniform concentration of Er3+ ions, the concentration of Er3+ ions at the end of the second region 104 (immediately before the interface of the second region 104 and the third region 106) is also C2.Similarly, the concentration of Er3+ ions at the beginning of the fourth region 108 (immediately after the interface of the interface of the third region 106 and the fourth region 108)is C3. Finally, the concentration of Er3+ ions at the beginning of the fifth region 110 (immediately after the interface of the fourth region 106 and the fifth region 110), is zero. It is to be noted that since the third region 106 is not doped with any rare-earth element based dopant, the concentration of Er3+ ions at the interfaces of the third region 106 with the second region 104 and fourth region 108 (and towards the third region 106) is zero. Relationship between concentration of rare-earth element ionswithinREDOF100is described below: 50×1023ions/m3 = C2= 1028ions/m3; 0= C1