CLIAMS:We claim:
1. A method of making an optical fiber preform comprising:
i. Preparing a first stack (200) of plurality of symmetrical and identical annular disks of a first type (202), each of said annular disks of said first type (202) having a refractive index µt, said first stack (200) being symmetrical about a first axis (206) and having an outer radius rtpf, said first stack (200) further including a first cylindrical aperture (204), said first cylindrical aperture (204) being symmetrical about said first axis(206) and having a radius ropf;

ii. Preparing a second stack (300) of plurality of symmetrical and identical annular disks of a second type (302), each of said annular disks of said second type (302) having a refractive index µcld, µcld being greater than µt, said second stack (302) being symmetrical about a second axis (306) and having an outer radius Rpf, said second stack (300) further including a second cylindrical aperture (304), said second cylindrical aperture (304) being symmetrical about said second axis (306) and having a radius rtpf;

iii. Preparing a cladding stack assembly (400) by placing said first stack(200) within said second cylindrical aperture(304) in a manner such that said first axis (206) and said second axis(306) are collinear;

iv. Preparing an complete assembly (600) by filling said first cylindrical aperture (204) included within said cladding stack assembly (400) by a filler structure (500), said filler structure (500) being symmetrical about a longitudinal axis (502), said filler structure (500) being placed within said first cylindrical aperture (204) in a manner such that said first axis (206) and said longitudinal axis (502) are collinear, said filler structure including either of:
a. at least one transparent cylindrical region having a refractive index µc, µc being greater than µcld, said at least one transparent cylindrical region symmetrically surrounding said longitudinal axis (502) of said filler structure (500); or
b. at least one cylindrical region, which when heated, would permanently become transparent and have a refractive index µc, µc being greater than µcld, said at least one cylindrical region symmetrically surrounding said longitudinal axis (502) of said filler structure (500); and
v. Heating and fusing said complete assembly (600) to obtain a solid transparent optical fiber preform.
2. The method as claimed in claim 1, wherein each of said annular disks of said first type is made of glass.
3. The method as claimed in claim 1, wherein each of said annular disks of said second type is made of glass.
4. The method as claimed in claim 1, wherein said filler structure is a solid glass rod.
5. The method as claimed in claim 1, wherein said filler structure is a stack of non-perforated glass disks.
6. The method as claimed in claim 1, wherein said filler structure is a stack of perforated glass disks.
7. The method as claimed in claim 1, wherein said filler structure includes a third stack of annular glass disks.
8. The method as claimed in claim 7, wherein said filler structure further includes a glass rod.
Dated: 31st Day of March, 2015 Signature
Arun Kishore Narasani Patent Agent ,TagSPECI:FIELD OF INVENTION
[001] The present invention relates to relates to a method of manufacturing optical fiber preforms. More particularly, the present invention relates to a method for manufacturing optical fiber performs for use in manufacturing bend-insensitive optical fiber.
BACKGROUND OF INVENTION
[002] Refractive index (referred as ‘RI’ hereinafter) profile of an optical fiber play a vital role in defining its optical properties. Optical fibers are manufactured by drawing glass/polymer fibers from transparent cylindrical structures (popularly known as optical fiber preform). Since, an optical fiber is a thinner and longitudinally extended version of its parent optical fiber preform, it can be fairly concluded that, RI profile of an optical fiber preform play a vital role in defining optical properties of an optical fiber drawn from it. In fact, to a major extent, the RI profile of an optical fiber is proportionately identical to the RI profile of its parent optical fiber preform. To ensure that optical properties of manufactured optical fiber meets desired expectations, it is desired that the RI profile of the parent optical fiber perform is an accurate proportionate replica of the desired target RI profile to be achieved in the optical fiber. Even a slight deviation from the desired target RI profile would show in the drawn optical fiber and can degrade or even spoil the entire product.
[003] In bend-insensitive optical fiber (BIOF), i.e. an optical fiber which includes a region of depressed refractive index sandwiched in its cladding (region of depressed refractive index is referred as ‘trench’ hereinafter), strict adherence to a target refractive index profile is necessary. To provide improved bend-insensitivity in a BIOF, among various desired features, it is also expected that a sharp change in refractive index occurs at the interfaces of the trench and the cladding.
[004] Accurate incorporation of a trench region with step changes in RI at its interfaces with the cladding, during manufacturing a parent optical fiber preform for the BIOF (hereinafter referred to as ‘BIOF preform’), is a very sensitive and delicate task. Since the RI profile of an optical fiber would be proportionately similar to the optical fiber preform from which it is drawn (or its parent optical fiber preform), it has to be ensured that an accurate trench gets incorporated within the BIOF preform itself. During manufacturing of a BIOF preform, even a slight deviation from the desired shape of the trench would show in the BIOF, and can degrade or even spoil the entire product. In conventional silica based BIOF manufacturing methods, Fluorine dopants are popularly used to create a region of depressed refractive index within cladding of the BIOF. However, since most of the popularly used Fluorine based dopants are volatile, it’s difficult to keep them confined within desired locations during manufacturing of the parent preform. For these and other method specific limitations, in most conventionally known methods for manufacturing optical fiber preforms such as OVD, VAD, MCVD, PCVD, and Sol-Gel (except Rod-in-cylinder or Rod-in-tube method), more or less, achievement of accurate step change in refractive index at the interface of trench and cladding is difficult.
[005] A major obstacle in achieving step changes in refractive index at the interface of trench and cladding within an optical fiber preform is undesired diffusion or migration of refractive index changing dopants (say Fluorine based dopants). As an example, during manufacturing of optical fiber preforms, unwanted diffusion or migration of refractive index changing dopants from their desired and intended locations within the preform to regions of the preform where their presence is not desired, tends to spoil desired shape of the trench and optical properties in the optical fiber drawn from the preform.
[006] Due to such undesired migration of dopants:
i. step changes in refractive index at the interface of trench and cladding within an optical fiber preform is practically never achieved, and
ii. the achieved optical properties of an optical fiber drawn from such preform differs from the intended target
[007] The problem in achieving accurate step index change in refractive index at the interface of trench and cladding becomes more severe when thickness of the sandwiched trench region is to be kept low. Under heavy migration, dopants may migrate to surrounding regions, and can hamper achievement of desired width of the trench. Additionally, such undesired migration would also reduce lowering of refractive index of trench to a desired level.
[008] It’s due to the above cited problems that efficient manufacturing of good quality BIOF preforms which would yield high quality BIOF remains a challenge. In such preforms it is desired that, at the interfaces of cladding and the trench, there be a sharp change in refractive index (or a step change as referred throughout herein). To form such a region, while manufacturing the preform, a desired specific portion of the preform is doped with Fluorine dopant. However, due phenomenon of undesired diffusion as described above, Fluorine dopant do not remain confined within its intended boundaries, and tend to diffuse in to surrounding regions of the preform. Due to this undesired diffusion, intended step change in refractive index is not achieved and at times, entire shape of the trench is lost. As a result of this deviation from achieving the desired target refractive index profile, desired bend-insensitivity and other optical parameters the optical fiber get adversely affected.
[009] The Rod-in-cylinder (referred as RIC hereinafter) or Rod-in-tube (referred as RIT hereinafter), method of making optical fiber preforms, is better at achieving an almost step change in refractive index at the interface of cladding and trench. In this method, as an example, a transparent glass rod is inserted in a transparent hollow tube (both rod and tube being made of glass having different refractive index), and the combined assembly is fused together to form a solid cylindrical glass body. The solid cylindrical glass body thus made, would have an almost step change in refractive index at the region where outer surface of the glass rod fused with the inner surface of the hollow tube. To manufacture a BIOF preform by RIC/RIT, a glass rod, the outer layer of which includes a layer having RI of the inner cladding of the BIOF is inserted in a first hollow tube made of glass having lower refractive index (say Fluorine doped glass the value of refractive index of which is equal to the refractive index of trench). The assembly of the glass rod and the tube is further placed in another glass tube, the RI of which is equal to the outer cladding of the BIOF to provide a complete assembly. Finally the complete assembly is heated and fused to form a transparent BIOF preform. The glass rod used in the above method includes a region which provides core of the BIOF. Further, the dimensions of each tube and the glass rods are taken in a proportional match with the dimensions of the portions of the target RI profile which they respectively represent. Since both rod and tube are made of solid non-porous glass, and since the dopants in either of rod or tube are locked in a fixed glass structure, undesired migration of dopants from either rod to tube or vice-versa does not occur.
[0010] Although RIC/RIT is better at providing almost step changes in refractive index at the interface of cladding and trench, and can be used for making good quality BIOF preforms, it however has some major drawbacks. Firstly, it includes a lot of precision constraints over its resources and secondly, commercial production of preforms made by RIC/RIT is expensive in comparison to other conventionally known methods.
[0011] As an example of drawbacks of RIT/RIC method, preparing a length of geometrically straight silica tube or rod of acceptable quality is a challenge in self. Even a slight bend in glass tube or rod may result in problems such as non-alignment with the rod (or tube as the case may be), and appearance of voids at the interface of rod and the tube. Moreover, preparing rods and tubes with precision standards as required comes with added costs. Still further, in order to maintain such high precision standards, a lot of resources (both tubes and rods) have to be discarded as unusable. Hence inefficient use or resources is another major drawback.
[0012] In summary, though RIC method is better at providing almost step changes in refractive index, it requires maintenance of high precision levels, added manufacturing costs and is commercially less viable.
[0013] Hence there’s an acute need for an improved method of manufacturing optical fiber preforms which:
i. would have step changes of refractive index at interface of cladding and trench
ii. would have lesser precision requirements
iii. would be simple to execute, and
iv. would be cost effective.
OBJECT OF INVENTION

[0014] It is an object of the present invention is to provide a method of manufacturing an optical fiber preform which would yield an optical fiber having step changes in refractive index at interfaces of cladding and trench.
[0015] It is an object of the present invention is to provide method of manufacturing bend-insensitive optical fiber preform which would have accurate step changes in refractive index at interfaces of trench and cladding, would have lesser precision requirements, would be simple to execute, and which be cost effective.
[0016] Another object of the present invention is to provide a simple method of manufacturing bend-insensitive optical fiber preforms.
[0017] Another object of the present invention is to provide a method of manufacturing bend-insensitive optical fiber preforms which would require less precision requirements than needed in conventionally known methods.
[0018] Yet another object of the present invention is to provide a cost effective method of manufacturing bend-insensitive optical fiber preforms.
[0019] These and other objects of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.
SUMMARY
[0020] Accordingly, the present invention provides a method for manufacturing optical fiber preforms which overcomes drawbacks of conventional methods and achieves objects of the invention.
[0021] In accordance with the present invention, for manufacturing a BIOF preform having a target RI profile, firstly, a first stack of plurality of identical and symmetrical annular disks of a first type, and second stack of plurality of identical and symmetrical annular disks of a second type are prepared. Each annular disk of the first type has refractive index µt, and each annular disk of the second type has refractive index µcld, µcld being greater than µt. The first stack and the second stack are both of same height. The first stack includes a first cylindrical aperture. First cylindrical aperture having a radius ropf. Both the first stack and the first cylindrical aperture are symmetrical about a first axis. The first axis passes through the first cylindrical aperture. The second stack includes a second cylindrical aperture. The Second cylindrical aperture having a radius rtpf. Both the second stack and the second cylindrical aperture are symmetrical about a second axis. The second axis passes through the second cylindrical aperture.
[0022] Thereafter, cladding stack assembly is prepared by placing the first stack within the second cylindrical aperture to provide a cladding stack assembly in a manner such that the first axis and the second axis align collinearly.
[0023] In the following step, a complete assembly is obtained by filling the first cylindrical aperture (lying within the cladding stack assembly) with a filler structure, said filler structure being symmetrical about a longitudinal axis, said filler structure being placed within the first cylindrical aperture in a manner such that the first axis and the longitudinal axis are collinear. Further, the filler structure includes either of:
i. at least one transparent cylindrical region having a refractive index µc, µc being greater than µcld, said at least one transparent cylindrical region surrounding a longitudinal axis of the filler structure; or
ii. at least one cylindrical region, which when heated, would permanently become transparent and have a refractive index µc, µc being greater than µcld, said at least one cylindrical region surrounding a longitudinal axis of the filler structure
[0024] Finally the complete assembly is heated and fused to form a consolidated BIOF preform.
[0025] It is ensured that the structure and dimensional values of physical parameters of each of the first stack, the second stack, and the filler structure are kept in mapping proportions with the structure and dimensions of regions of the target RI profile which they would respectively provide.
[0026] In the consolidated BIOF preform hence obtained, trench (or region of depressed index) is provided by the first stack, and at least a portion of the outer cladding (portion of cladding surrounding the trench and lying adjacent to trench) is provided by the second stack. Further, the core of the preform and the inner cladding is provided by the filler structure.
[0027] In one embodiment of the invention in which, it is desired to prepare a BIOF preform which would be used to obtain a BIOF of circular cross-section and having the following structure:
1. a core surrounding a longitudinal axis of the optical fiber, said core having a refractive index µc
2. an inner cladding, said inner cladding surrounding said core and lying adjacent to, said inner cladding having a refractive index µcld, µcld being less than µc
3. a trench (region of depressed refractive index) surrounding said second inner cladding and lying adjacent to it, said trench having a refractive index µt, µt being less than µcld
4. an outer cladding surrounding said trench and lying adjacent to it, said second outer cladding having a refractive index µcld, µcld being less than µc
[0028] The filler structure used for filling the master central aperture is a glass rod having an outer diameter ‘dr’. The glass rod would provide for the core and the inner cladding for desired BIOF preform (and the fiber drawn from it).The glass rod is symmetrical about a longitudinal axis and has a circular cross-section. Within a cross-section of the glass rod, a region extending to a radial distance rcpf around the center of the cross-section (2×rcpfbeing equal to (dr/2)), represents the region within the rod which has a refractive index µc, and which would provide core of the optical fiber. Remaining portion of the cross-section (which lies outside the radial distance rcpf) represents the portion of the glass rod which would provide the inner cladding (having a refractive index µcld) of the BIOF preform. When placed in the first cylindrical aperture, longitudinal axis of the glass rod becomes collinear with the first, and the second axis.
[0029] It is ensured that the structure and dimensional values of physical parameters of each of the first stack, the second stack, and the filler structure are kept in mapping proportions with the dimensions of regions of the target RI profile which they would respectively provide.
[0030] Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.
BRIEF DESCRIPTION OF FIGURES
[0031] FIG. 1 illustrates a target Refractive index profile of a BIOF, a preform for which would be prepared in accordance a first embodiment of the present invention.
[0032] FIGS. 2A-2C, FIGS. 3A-3C, FIGS. 4A-4B, FIG. 5A-5B, FIGS. 6A-6C illustrate the process of making BIOF preform in accordance with the first embodiment of the present invention
[0033] It should be noted that the accompanying figures are intended to provide a better understanding of embodiment of the present invention, and are not intended to limit the scope of the present invention. It should also be noted that accompanying figures are for the purpose of illustration only and are not necessarily drawn to scale. For example, the size of some of the components in the figures may be exaggerated, relative to others in order to improve the understanding of the present invention. Accompanying figures are intended only to provide a clear understanding of the embodiment of the present invention. 
DETAILED DESCRIPTION OF INVENTION
[0034] Accordingly, the present invention provides a method for manufacturing optical fiber preforms which overcomes drawbacks of conventional methods and achieves objects of the invention. It is to be noted that though the following detailed description describes an embodiment of the present the present invention for manufacturing BIOF preforms, scope of the present invention does not gets limited to manufacturing BIOF preforms only. In fact, all embodiments of methods of manufacturing optical fiber preform which include a trench (or a region of depressed refractive index), and which are in accordance with the present invention are fully covered within the scope of the present invention.
[0035] The present invention provides a method for manufacturing a BIOF preform which would yield a BIOF having a region of depressed RI (hereinafter referred as ‘trench’) included in its cladding. The BIOF preforms as provided by the method of the present invention would necessarily include:
1. A core region which would be used to provide a path within the BIOF for optical signals to travel. The core region would have a refractive index µc.
2. An inner cladding region which would surround the core region and would lie adjacent to it. The inner cladding region having a refractive index µcld, µcld being less than µc
3. A region of depressed refractive index (commonly known as ‘trench’) surrounding the inner cladding region. The region of depressed RI (or the ‘trench’) having a refractive index µt, µt being less than µcld
4. An outer cladding region which would surround the region of depressed refractive index and would lie adjacent to it. The outer cladding region having a refractive index µcld, µcld being less than µc
[0036] It is to be noted that:
1. throughout the text which follows hereinafter, the term ‘BIOF’ represents a bend-insensitive optical fiber
2. throughout the text which follows hereinafter, the term BIOF preform represents an optical fiber preform which would yield a BIOF
3. throughout the text which follows hereinafter, the term ‘trench’ has been used to represent the region of depressed refractive index within in the BIOF preform or the BIOF, or the RI profile of the BIOF
4. throughout the text which follows hereinafter, the term ‘inner cladding’ has been used to represent the portion of cladding which lies adjacent to the trench and is located between the core and the trench
5. throughout the text which follows hereinafter, the term ‘outer cladding’ has been used to represent the portion of cladding which lies adjacent to the trench and is not located between the core and the trench
[0037] The method of manufacturing BIOF preform, in accordance with the present invention, ensures accurate formation of trench within the BIOF preform. In accordance with the present invention, in order to manufacture a BIOF preform which would yield a BIOF having desired optical parameters, firstly, a target RI profile of the BIOF preform (or of the BIOF fiber to be drawn from it) is studied, and values of refractive index and dimensions of various regions of the BIOF in accordance with the RI profile are noted. Thereafter, in accordance with an embodiment of the invention, a first stack of annular disks of the first type is prepared. Annular disks of the first type are symmetrical in shape, identical to each other, and are made of glass whose RI is equal to desired value of RI of the trench of the target RI profile (i.e. µt). The first stack would provide the trench region in the target BIOF preform to be manufactured. The first stack is a symmetrical around a first axis, is cylindrical in shape and includes a first cylindrical aperture. Thereafter, to provide for at least a portion of the outer cladding regions of the BIOF preform, the first stack is placed within a second stack of made of annular disks of a second type to provide a cladding stack assembly. Annular disks of the second type are symmetrical in shape, identical to each other, and are made of glass whose RI is equal to desired value of RI of the outer cladding of the target RI profile (i.e. µcld). The second stack is symmetrical around a second axis, is cylindrical in shape, and includes a second cylindrical aperture. Heights of the first stack and the second stack are identical. The dimensions of second cylindrical aperture are selected in a manner such that it provides enough space for the first stack to fit in.
[0038] It is to be noted that, within the cladding stack assembly of the first stack, and the second stack (assembled in a manner as described above), both the first axis and the second axis are collinear, and the second stack which circumscribes the first stack provides for at least a portion of the outer cladding. Once both the first stack and the second stack are assembled together as described, one among various possible filler structures which would provide other regions of the BIOF (such as core, inner cladding, and/or additional layers of the outer cladding, if any) is inserted in the first cylindrical aperture (which now becomes a part of the cladding stack assembly). The filler structure being used is symmetrical about a longitudinal axis, said filler structure being placed within the first cylindrical aperture in a manner such that the first axis and the longitudinal axis are collinear. The filler structure being used includes either of:
i. at least one transparent cylindrical region having a refractive index µc, µc being greater than µcld, said at least one transparent cylindrical region surrounding a longitudinal axis of the filler structure; or
ii. at least one cylindrical region, which when heated, would permanently become transparent and have a refractive index µc, µc being greater than µcld, said at least one cylindrical region surrounding a longitudinal axis of the filler structure
[0039] As an example, a glass rod which would provide core and inner cladding of the optical fiber may be inserted in the first cylindrical aperture of the cladding stack assembly to provide a complete assembly. It is ensured that the structure and dimensional values of physical parameters of each of the first stack, the second stack and the glass rod (including the regions included in it) are kept in mapping proportions with the dimensions of regions of the target RI profile which they would respectively provide.
[0040] Finally, the complete assembly is heated (to about a temperature of about 2000°C since each of the annular discs and the glass rod are made of glass)_and fused to form a solid transparent body, the BIOF preform.
[0041] It is to be noted that the glass rod (including the regions included in it), and annular disks of each of the first stack and the second stack are made such that their Refractive index is equal to the region of the target RI profile which they (or their respective stacks) would represent. Similarly, all dimensions of the glass rod (including the regions included in it), and annular disks of each of first stack and the second stack are taken in proportion to dimensions of portions which they represent in the target refractive index profile. Heights of the glass rod, the first stack and the second stack were kept same.
[0042] The invention is described in detail below in conjunction with the accompanying figures. It is to be noted that terminology used throughout the specification and claims herein are given their ordinary meanings. Those with ordinary skill in the art will appreciate that the elements in the figures are illustrated for simplicity and clarity. There may be additional components described in the foregoing application that are not depicted on one of the described drawings. In the event such a component is described, but not depicted in a drawing, the absence of such a drawing should not be considered as an omission of such design from the specification.
[0043] As required, detailed embodiment of the present invention is disclosed herein; however, it is to be understood that the disclosed embodiment is exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.
[0044] While the specification concludes with the claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawings, in which like reference numerals are carried forward.
[0045] It should be noted that the terms "first", "second", “third” and the like, herein do not denote any order, ranking, magnitude, or order of 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.
[0046] Reference will now be made in detail to an embodiment of the present invention in conjunction with accompanying figures. This embodiment provides a method for making a BIOF preform which would yield a BIOF having a target refractive index profile as illustrated in FIG. 1.
[0047] FIG. 1 provides an illustration of a desired refractive index profile of a BIOF 100, the BIOF preform for which would be prepared by the first embodiment of the present invention. In figure FIG.1, refractive index ‘µ’ is plotted against radial distance ‘r’ from the center102 of a cross-section of the BIOF 100, said cross-section being taken along a plane perpendicular to a longitudinal axis of the BIOF 100. As illustrated the, BIOF 100 has an outer radius ‘R’, and further includes:
i. a core region 104 extending from center 102 of the cross-section of the BIOF 100 to a radial distance ‘rc’. The core region 104 has a refractive index ‘µc’
ii. an inner cladding region 106 extending from radial distance ‘rc’ to a radial distance ‘ro’, ‘ro’ being greater that ‘rc’. The inner cladding region 106 having a refractive index ‘µcld’. µcld being less than µc.
iii. a trench region 108 extending from radial distance ‘ro’ to a radial distance ‘rt’, ‘rt’ being greater that ‘ro’. The trench region 108 having a refractive index ‘µt’. µt being less than µcld.
iv. an outer cladding region 110extending from radial distance ‘rt’ to a radial distance ‘R’. The outer cladding region 110 has a refractive index ‘µcld’. As mentioned above, µcld is less than µc.
[0048] In accordance with the current embodiment of the present invention for making BIOF preform which would yield the BIOF 100, in the first step, as illustrated in FIG. 2A, a first stack200 is prepared by stacking multiple symmetrical and identical annular disks of a first type202. First stack200is a cylindrical structure and further includes a first cylindrical aperture204. The first stack200 and the first cylindrical aperture204 are symmetrical about a first axis 206. First axis 206 passes through the first cylindrical aperture204. Further, each of the annular disks of the first type 202 (which are stacked to form the first stack 200) are made of glass having a refractive index ‘µt’, ‘µt’ being the value of refractive index of the trench of the target RI profile of BIOF 100.First stack200 would provide the trench region 106 of the BIOF 100. An illustration of one of the annular disks of the first type 202 as described, is provided in FIG. 2Band FIG. 2C.While FIG. 2Billustrates a three dimensional view of one of the annular disks of the first type 202, FIG. 2C illustrates a top view of one of the annular disks of the first type 202. FIGS. 2A-2C also illustrates dimensional parameters of first stack200 and one of the annular disks of the first type 202. As illustrated, the outer radius and inner radius of each of the annular disks of the first type 202 is rtpf and ropf respectively. It is obvious that the diameter of the first cylindrical aperture 204 is equal to 2×ropf. Also height of each of the annular disks of the first type 202 is illustrated as hf. Height of the first stack 200 is illustrated as H. It is to be noted that for simplicity and ease of understanding, the actual number of annular disks of the first type 202 used in first stack 200 is not illustrated in FIG. 2A. However, the number of annular disks of the first type 202, i.e. ‘Nf’, used in first stack 200 can be calculated by Nf= H/hf.
[0049] In the next step, portions which would provide for the complete outer cladding 110 of the BIOF were prepared. For preparing portions which would provide for outer cladding 110, a second stack 300 is prepared by stacking multiple symmetrical and identical annular disks of a second type 302. Second stack 300 is a cylindrical structure and further includes a second cylindrical aperture 304. The second stack300 and the second cylindrical aperture 304 are symmetrical about a second axis 306.Second axis 306 passes through the second cylindrical aperture 304.Further, each of the annular disks of the second type302 (which are stacked to form the first stack 300) is made of glass having a refractive index ‘µcld’, ‘µcld’ being the value of refractive index of the outer cladding of the target RI profile of BIOF 100. Second stack300 would provide the outer cladding region 110of the BIOF. An illustration of one of the annular disks of the second type 302 as described, is provided in FIG. 3B and FIG. 3C. While FIG. 3B illustrates a three dimensional view of one of the annular disks of the second type 302, FIG. 3C illustrates a top view of one of the annular disks of the second type 302. FIGS. 3A-3C also illustrates dimensional parameters of second stack300 and one of the annular disks of the second type 302. As illustrated, the outer radius and inner radius of each of the annular disks of the second type 302 is Rpf and rtpf respectively. It is obvious that the diameter of the second cylindrical aperture 304 is equal to 2×rtpf. Also height of ach of the annular disks of the second type 302 is illustrated as hocld. Height of the second stack 300 is same as the height of the first stack 200 and is illustrated as H. It is to be noted that for simplicity and ease of understanding, the actual number of annular disks of the second type 302 used in second stack 300 is not illustrated in FIG. 3A. However, the number of annular disks of the second type 302, i.e. ‘Ns’, used in second stack 300 can be calculated by Ns= H/hocld.
In the next step, both stacks i.e. the first stack 200and the second stack 300are assembled in a manner such that:
i. The first axis 206and the second axis 306 become collinear, and
ii. the first stack200isinserted in the second cylindrical aperture304
[0050] The process of assembling the first stack200 and the second stack300 as described above is illustrated in FIG. 4A and 4B. The combined assembly of the first stack200 and the second stack300 prepared in a manner as described above is illustrated in FIG. 4Bas cladding stack assembly 400. It is to be noted that throughout the text provided hereinafter, the combined assembly of the first stack200 and the second stack300 prepared in a manner as described above would be referred as cladding stack assembly 400. The cladding stack assembly 400 is symmetrical about an axis 402 (within the cladding stack assembly 400, axis 402 is collinear with the first axis 206 and the second axis 306). It is also to be noted that the first cylindrical aperture204 remained intact in the cladding stack assembly 400. The cladding stack assembly 400would provide for the trench region 108and complete outer cladding 110of the BIOF100.
[0051] In the following step, a filler structure, which would provide for the core 104and the inner cladding106of the BIOF 100, was combined with the cladding stack assembly400. In the present embodiment, a glass rod 500 was used as a filler structure. The core 104and the inner cladding 106of the BIOF 100 were provided by the glass rod 500.
[0052] FIG. 5Aillustrates a view of the glass rod 500. The glass rod 500is a cylindrical body which is symmetrical about a longitudinal axis 502.FIG. 5B illustrates a magnified cross-sectional view of the glass rod 500 which was used to provide the core 104 and the inner cladding 106 of the BIOF 100. The cross-section as illustrated in FIG. 5Bis circular in shape and is taken along a plane which is perpendicular to the longitudinal axis 502. The glass rod 500has a radius of ropf (so diameter of glass rod 500 is 2 × ropf, it is as illustrated in FIG. 5B as ‘2(ropf)’). Further, the glass rod 500 includes following:
i. rod core region 504, and
ii. rod cladding region 506

[0053] The rod core region504 is circular in shape, and lies symmetrically around a centre508 of the cross-section of the glass rod 500. Rod core region 504 extends up to a radial distance ‘rcpf’ from center508, and has a refractive index ‘µc’ which is equal to the refractive index of core region 104 of the BIOF 100(as illustrated in FIG. 1).The rod cladding region 506surrounds the rod core region 504, and extends to an outer periphery510 of the glass rod 500. Thickness of ‘t’ the rod cladding region 506 is calculated as t= (ropf-rcpf). Longitudinal axis 502 passes through the rod core region 504 and rod core region 504 symmetrically surround longitudinal axis 502.The rod cladding region 506lies adjacent to the rod core region504 and lies symmetrically around the longitudinal axis 502.It is to be noted that the rod core region 504would provide core 104 of the BIOF 100, and rod cladding region 506 would provide for the inner cladding 106 of the BIOF 100.
[0054] The glass rod 500 as described above was inserted in the cladding stack assembly 400to provide a complete assembly600. Within the cladding stack assembly 400,the core rod500occupies the space within the void of first cylindrical aperture204. FIGS. 6A,6Band6C illustrate the process of preparing the complete assembly600as described. The complete stack assembly 600 is illustrated in FIG. 6C.
[0055] TABLE 1 below illustrates values of dimensional parameters and refractive index of various components which were used for manufacturing the BIOF preform in accordance with the first embodiment of the invention as described.
TABLE 1
S. No. Dimensional Parameters Values/Magnitude of Dimensional Parameters
1 µc 1.449
2 µcld 1.444
3 µt 1.440
4 ro 4 microns (4×10-6 meter)
5 rc 8 microns (8×10-6 meter)
6 rt 16 microns (16×10-6 meter)
7 R 62.5 microns (62.5×10-6 meter)
8 ropf 4 millimeter (4×10-3 meter)
10 rcpf 8 millimeter (8×10-3 meter)
11 rtpf 16 millimeter (16×10-3 meter)
12 Rpf 62.5 millimeter (62.5×10-3 meter)
13 H 600 millimeter (600×10-3 meter)
14 ht 10 millimeter (7×10-3 meter)
15 hocld 15 millimeter (15×10-3 meter)

[0056] Since the RI profile of an optical fiber is a proportional replica of the RI profile of its parent optical fiber preform, it would observed, through data provided in TABLE 1, that the dimensional parameters of the annular disks of the first type 202 and annular disks of the second type 302 (or of the first stack 200 and the second stack 300) and the glass rod 500 were chosen to be a proportional replica of the dimensions of various regions they would represent in the BIOF 100. It would be observed that in comparison to target RI profile of BIOF 100 as illustrated in FIG. 1, the corresponding dimensions of the components of the BIOF preform being manufactured were scaled by a factor of 1000.Further, the number of annular disks used in each of the first stack 200 and the second stack 300 were 60 and 40 respectively.
[0057] Finally, the complete assembly 600 was heated in a furnace to about a temperature of about 2000°C to fuse all its constituent components together to yield a transparent cylindrical glass preform, which is the desired BIOF preform. In a further processing step, one of the ends of the BIOF preform was processed and shaped in to a cone. The transparent cylindrical glass preform was transferred in to an optical fiber draw tower to obtain desired BIOF. Within the draw tower set-up, the drawn optical fiber was also covered with two layers of polymer coatings for providing physical protection and further enhancing its optical performance. The RI profile of the drawn BIOF was found to match with the target RI profile illustrated in FIG. 1.
[0058] It is to be noted that, in the current embodiment, for enabling easy and simple understanding, the exact count of annular disks in each of the first stack 200 and the second stack 300 is not illustrated in FIG. 2A and FIG. 3A.The number of annular disks illustrated in figures illustrating the first stack 200 or the second stack 300 (i.e. FIG. 2A and FIG. 3A) are provided only for enabling easy and simple understanding of the invention. Scope of the present invention also does not gets limited by the number of annular disks employed and the height of each of the annular disks in either the first stack 200 or the second stack 300.Scope of the present invention also does not gets limited by height of either the first stack 200 or the second stack 300. In embodiments of the present invention which employ different heights of the first stack 200 and the second stack 300, portion of the complete assembly of the stacks of annular disks, and the filler structure which would provide the core region, the inner cladding region, the trench region and at least a portion of the outer cladding region would provide for the desired BIOF preform.
[0059] The scope of the present invention also does not gets limited by the method of manufacturing the annular disks for any of the stacks. For example, in the first embodiment of the invention described above, the method with which the annular disks of the first type202, or the annular disks of the second type 302 were manufactured, does not affect the scope and coverage of the invention. In other words, all embodiments of methods of manufacturing an optical fiber preform, which are in accordance with the present invention, are fully covered within the scope of the present invention regardless of the method by which the annular disks, being used in them, are made.
[0060] Popularly known methods of manufacturing the annular disks (say, annular disks of the first type 202, or the annular disks of the second type 302 as described in the first embodiment of the invention above) include the following:
i. Preparing annular disks by pouring molten glass in a mold
ii. Preparing annular disks by cutting a hollow cylindrical glass tube along a plane which is perpendicular to a longitudinal axis of the tube
iii. Preparing annular disks from a glass plate by removal of selected portions of glass
iv. Preparing annular disks by cutting a solid glass cylinder and then making an aperture through each of them
[0061] It is to be understood that the scope of the present invention also does not gets limited by the type and amount of dopants used to provide desired refractive index values to any of the annular disks which are used for manufacturing an optical fiber preform. In fact, scope of the present invention also does not gets limited by the material composition of any of the annular disks which are used for manufacturing an optical fiber preform. All embodiments of method of manufacturing optical fiber preform which are in accordance with the present invention are fully covered within scope of the present invention regardless of the material composition of any of the annular disks which are used for manufacturing an optical fiber preform.
[0062] It is to be noted that the scope of the present invention also does not gets limited by the type of filler structure being used. As an example, in another embodiment of the present invention, a stack of non-perforated glass disks can be used as a filler structure. In yet another embodiment, a stack of glass cylinders can be used as filler structure. Further, in another embodiment of the present invention, the filler structure can be another assembly of multiple stacks of disks. Such an assembly would include one or more stacks of annular disks. In yet another embodiment the filler structure can be a combination of one or more stacks of annular disks and a glass rod. In yet another embodiment, the filler structure could be a porous rod of Silica soot which would include a cylindrical region of Germanium-doped Silica soot. The cylindrical region of Germanium-doped Silica soot lying symmetrically around a longitudinal axis of the rod. The Ge-doped region would further be surrounded by a region of undoped Silica soot. The region of undoped Silica soot having a uniform thickness. In this embodiment, when the complete assembly is heated to sintering temperature (say ~1550°C), the cylindrical region of Germanium-doped Silica soot would get converted in to solid transparent glass which would provide core of the BIOF, and the region of undoped Silica soot would get converted in to solid transparent glass which would provide the inner cladding of the BIOF. It is to be noted that the dopant concentration of the Germanium-doped region is selected such that after getting converted in to solid transparent glass, the Germanium-doped region would also attain desired refractive index value of the core of the BIOF. Technically, the type of filler structure being used would, to a large extent, depend on the requirements of the number and type of regions (and RI of each if these regions) which are intended to be included and surrounded by the trench region of the BIOF. It is also to be noted that all embodiments of methods of making BIOF preform which are in accordance with the present invention and which employ any of the filler structure described above (or their modifications) are fully covered within the scope of the present invention.
[0063] Further, it is to be understood that various modifications in the first embodiment of the invention described above would be obvious to a person skilled in the art. It is to be noted that all such methods of manufacturing optical fiber preform which are in accordance with the present invention, and which are obvious modifications in the embodiments of the present invention are also fully covered within the scope of the present invention. As an example, in the first embodiment of the invention described above, if required, the cladding stack assembly 400 may be further be surrounded by one or more additional stacks of annular disks (each of the additional stacks providing additional regions of the outer cladding of the BIOF preform, and each of the additional stacks may have different refractive index). It is to be understood that a method of manufacturing BIOF preform by further surrounding the cladding stack assembly 400(as described above) with one or more other structures (such as by placing the cladding stack assembly 400 within another stack of annular disks or by placing the cladding stack assembly 400 within a tube) is also fully covered within the scope of the present invention.
[0064] The present invention also facilitates an advantage of efficient resource utilization in comparison to RIC/RIT method. As mentioned earlier, RIC/RIT method has a mandatory requirement of geometrically straight hollow tubes. In the process of meeting such strict requirement, a lot of hollow tubes get rejected simply because they are not acceptably straight for being used in RIC/RIT method. The present invention however ensures that at least a portion of such tubes also get utilized. Since disks for the present invention can also be prepared by cutting hollow tubes cross-sectionally. All tubes (or at least a portion of such tubes) which got rejected simply because they were not acceptably straight for being used in RIC/RIT method, can now be used to manufacture annular disks for the present invention. Hence, the present invention also supports efficient resource utilization.
[0065] Though the embodiment of present invention is described by employing glass or silica annular disks, it is to be understood that embodiments of the invention which employ annular disks of different material (for example: polymer or plastic or silica soot) are also fully covered under the scope of the current invention. In other words, scope of the present invention does not gets limited by the material with which the annular disks are made of.
[0066] Scope of the present invention also does not get limited by the application of the BIOF preform produced by the present invention. As an example, instead of drawing BIOF, the BIOF preform manufactured in accordance with the present invention may further be stretched to yield glass rods. Such glass rods may further be processed and used for preparing optical fiber preforms by conventional methods (such as OVD and VAD). It is to be noted that regardless of the use, all embodiments of manufacturing BIOF preform which are in accordance with the present invention are fully covered within the scope of the present invention.
[0067] It would be obvious for a person skilled in the art to make/suggest/propose many more embodiment of the invention with minor or obvious modifications. It should be noted that all embodiments of the method of making optical fiber preform which have essential features of the invention incorporated within are fully covered within the scope of the present invention. In other words all embodiments of methods of manufacturing a BIOF preform, which are in accordance with the present invention, are fully covered within the scope of the present invention.
[0068] It is to be understood that the foregoing description is intended to illustrate and not limit the scope of the invention. Accordingly, the embodiment of the invention described above is not intended to limit the invention. Although described in the context of above embodiment, other embodiments of the invention which would be apparent to those skilled in the art are very well covered within the scope of the invention. Thus, while the invention has been particularly shown and described with respect to the above mentioned embodiment, it will be understood by those skilled in the art that certain modifications or changes may be made therein without departing from the scope and spirit of the invention.