The present disclosure relates to the field of manufacturing of a preform, and more particularly, relates to a system and a method of variable sintering of a soot blank.
BACKGROUND
[0002] Over the last few years, optical fibers are widely used for network communication due to enormous benefits over metal wires. The increasing demand of the optical fibers leads to mass production of preforms used for fabrication of the optical fibers. Sintering is used to manufacture the preforms. Typically, sintering is used to close pores that have been formed on a soot blank due to clad deposition, hence producing a sintered glass preform. The conventional method of sintering preforms produces the preforms having large diameter variations. The diameter variation in the preforms exists due to the presence of shear stress which causes over-stretching of glass preform during sintering. There exists prior-arts that measure the diameter variation in an optical fiber and control the diameter variation on the basis of measuring tension while drawing the optical fiber. A patent publication number EP089341515 discloses a device having a sensor to control speed for drawing the optical fiber to control the diameter variation within the optical fiber itself. However, the conventional techniques and prior-arts do not provide a convenient and cost-efficient method to control the diameter variation in the soot blank during the sintering process to form a superior physical quality preform, where the diameter variation along its length is less than 4 millimeter.
[0003] In light of the above stated discussion, there is a need for such a method of sintering soot blank that can overcome the above stated disadvantages.
OBJECT OF THE DISCLOSURE
[0004] A primary object of the present disclosure is to provide a system and method of variable sintering of a soot blank.

[0005] Another object of the present disclosure is to manufacture a sintered glass preform having a diameter variation in a range of 1-4 mm along a length of the sintered glass preform.
[0006] Another object of the present disclosure is to provide a method to reduce cost of manufacturing the sintered glass preform.
SUMMARY
[0007] In an aspect, the present disclosure provides a system of variable sintering of a soot blank. The soot blank is a cylindrical silica soot body. Variable sintering is opted since there exist diameter variation along the length of preform because during sintering viscous creep and shear force generates. The variable sintering is conducted under a controlled atmosphere of helium and chlorine gases at a flow rate of 30 slpm and 1 slpm respectively. The system of variable sintering of the soot blank applies heating the soot blank in a heating furnace (or sintering furnace). The sintering furnace is provided in a sintering tower. The sintering furnace has one or more coils (heating coils) having a length about 300mm that creates an effective heating zone for the sintering process. The soot blank has a plurality of soot blank zones along a longitudinal axis of the soot blank. The plurality of soot blank zones includes a bottom conical zone, a middle zone and a top conical zone. Each of the plurality of soot blank zones is characterized by a plurality of parameters. The plurality of parameters includes a sintering temperature, a feed rate and length of the soot blank zone. Excluding the top conical zone, each zone is sintered at a predefined plurality of sintering temperatures and feed rates. The middle zone implies the zone that is in between the top conical zone and the bottom conical zone. The middle zone is further divided into an upper middle zone and a lower middle zone. In between the upper middle zone and the lower middle zone, there are a plurality of sub-zones. The upper middle zone of the soot blank has a lower value of a sintering temperature and feed rate as compared to the lower middle zone and the bottom conical zone to prevent formation of viscous creep and overstretching of a sintered glass preform. That is, each zone of the plurality of soot blank zones undergoes the

optimized sintering process at the predefined plurality of sintering temperatures and feed rates to form the sintered glass preform by avoiding a viscous creep and overstretching. The temperature range for sintering the soot blank is 1520-1600 degree Celsius.
[0008] The system of the variable sintering enables a relative motion that takes place vertically between the soot blank and the effective heating zone because the soot blank is suspended vertically inside the sintering furnace with the help of a coupler. The relative motion between the soot blank and the effective heating zone enables sequential sintering of the plurality of soot blank zones.
[0009] In another aspect, the present disclosure provides a method of variable sintering of the soot blank. The method includes suspending the soot blank vertically in the sintering tower. The suspension of the soot blank is done through the coupler. The method further includes heating of the soot blank in the sintering furnace of the sintering tower to initiate the process of sintering. The next step is to have a relative motion between the soot blank and an effective heating zone so that each of the plurality of soot blank zones can undergo sintering but not at same time. Since the soot blank is suspended vertically, the relative motion will be vertically between the soot blank and the sintering furnace. The method further includes sequential sintering of the plurality of soot blank zones. The sequential sintering is due to the relative motion between the soot blank and the effective heating zone. Further, the method includes sintering the bottom conical zone of the soot blank with a high feed rate and sintering temperature, where eventually remaining zones of the plurality of soot blank zones will get sintered one by one but not at same time and at the predefined plurality of sintering temperatures and feed rates due to difference in the radial density profile. The last step is to sinter the upper middle zone of the soot blank, where it is cylindrical in shape that implies exclusion of the top conical zone to prevent the coupler from deteriorating during sintering. The feed rate of the middle zone of the soot blank will be lower than the feed rate of the bottom conical zone so as to prevent overstretching and formation of viscous creep into the soot blank.

[0010] These and other aspects herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawing. It should be understood, however, that the following descriptions are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention herein without departing from the spirit thereof.
BRIEF DESCRIPTION OF FIGURES
[0011] The invention is illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the drawings. The invention herein will be better understood from the following description with reference to the drawings, in which:
[0012] FIG. 1 illustrates a conventional sintered glass preform manufactured out of a soot blank in a sintering furnace.
[0013] FIG. 2 illustrates the soot blank having a plurality of soot blank zones before sintering in accordance with the present disclosure.
[0014] FIG. 3 illustrates the sintering furnace with one or more coils (heating coils) having a parabolic temperature profile in accordance with the present disclosure.
[0015] FIG. 4 illustrates an example of a diameter variation of the sintered glass preform with respect to the soot blank in accordance with the present disclosure.
[0016] FIG. 5 illustrates a coupler holding the soot preform vertically for sintering in accordance with the present disclosure.
[0017] FIG. 6 is a flow chart illustrating a method of variable sintering of the soot blank in accordance with the present disclosure.
DETAILED DESCRIPTION
[0018] In the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be obvious to a person skilled in the art that the

invention may be practiced with or without these specific details. In other instances, well known methods, procedures and components have not been described in details so as not to unnecessarily obscure aspects of the invention.
[0019] Furthermore, it will be clear that the invention is not limited to these alternatives only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art, without parting from the scope of the invention.
[0020] The accompanying drawings are used to help easily understand various technical features and it should be understood that the alternatives presented herein are not limited by the accompanying drawings. As such, the present disclosure should be construed to extend to any alterations, equivalents and substitutes in addition to those which are particularly set out in the accompanying drawings. Although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another.
[0021] FIG. 1 illustrates a typical sintered glass preform (102) manufactured from a soot blank (101) in a sintering furnace (301). The soot blank (101) is a cylindrical porous body made of silica particles. The soot blank (101) is heated in the sintering furnace (301) to form the sintered glass preform (102) made of solid silica glass. Herein, the soot blank (101) material, made of loose aggregate silica particles, is subjected to a high temperature and pressure to convert into the sintered glass preform (102), made of compact and solid silica glass. The sintered glass preform is used to manufacture optical fibers. In general, the optical fibers are a thin flexible fiber that are used for transmission of information as light pulses. Generally, the optical fibers are used in telecommunications to transmit telephone signals, internet communication, cable television signals and the like.
[0022] Commonly, the sintering is referred to as a heat treatment process in which a large quantity of loose aggregate material is subjected to a high temperature and pressure. The high temperature and pressure cause a loose material to become a compact solid piece. Further, the amount of heat

administered during the sintering is slightly less than a material's melting point. The sintering is performed in the sintering furnace present in a sintering tower. In one example of the present disclosure, the sintering may be performed in a controlled atmosphere of Helium and Chlorine gases. The helium gas provides an effective collapsing of pores formed on the soot blank due to clad deposition process into the soot blank and the chlorine gas is used for dehydroxylation process for removal of moisture from the soot blank. In an example, the sintering may be performed in the controlled atmosphere of Helium and Chlorine gases at 30 slpm and 1 slpm respectively. The helium gas is expensive, hence beyond 30 slpm, a variable sintering of the present disclosure will be expensive. Further, below 1 slpm of the chlorine gas, removal of moisture will not take place and beyond 1 slpm, the cost of the variable sintering will be increased.
[0023] In accordance with the present disclosure, FIG. 2 illustrates the
soot blank (101) having a plurality of soot blank zones (103, 104 105, 106)
along a longitudinal axis (107) that will undergo the process of the variable sintering. The variable sintering implies a process of sintering whereby different zones of the soot blank are sintered at a predefined plurality of sintering temperatures and feed rates. The longitudinal axis (107) is an imaginary line (one of the anatomical reference axes) running down the centre of the soot blank (101) perpendicular to a transverse plane, around which rotations in the transverse plane (for example, a body spin during a pirouette) occur. The plurality of soot blank
zones or portions includes a bottom conical zone (103), a middle zone (104, ,
105) and a top conical zone (106). The middle zone is a zone between the top conical zone (106) and the bottom conical zone (103). The middle zone is further partitioned into an upper middle zone (105) and a lower middle zone (104). The upper middle zone (105) is just below the top conical zone (106), while the lower middle zone (104) is just above the bottom conical zone (103). In between the upper middle zone (105) and the lower middle zone (104), there are a plurality of sub-zones. Number of the plurality of sub-zones may be 7. Alternatively, the number of the plurality of sub-zone may vary. The soot blank (101) may have a

weight less than 30-40 kg. Alternatively, the soot blank (101) may have the weight more than 50-60 kg. Alternatively, the weight of the soot blank may vary.
[0024] Each of the plurality of soot blank zones is characterized by a plurality of parameters. The plurality of parameters may include a sintering temperature, a feed rate and a length of the soot blank zone. The feed rate is a velocity at which the soot blank is fed, that is, advanced against the sintering
furnace (301). The plurality of soot blank zones (103, 104, , 105, 106) is fed
at a plurality of feed rates in an effective heating zone (303). Each of the plurality of soot blank zones may have different parameters.
[0025] FIG. 3 illustrates the sintering furnace (301) with one or more coils (heating coils) (302). The one or more coils (302) creates a heating zone (or the effective heating zone) (303) that is defined on the basis of a parabolic temperature profile. A majority of heat absorption by the soot blank (101) is accomplished in the effective heating zone (303). The effective heating zone (303) has a zone length less than the length of the soot blank (101). In an example, the one or more coils (302) has a length (or a coil length) of about 300 mm to create the effective heating zone (303) for the variable sintering. In another example, the one or more coils (302) has the length of about 400 mm to create the effective heating zone for the variable sintering. In yet another example, the one or more coils (302) has the length in a range of about 300-400 mm to create the effective heating zone on the basis of the temperature profile of the sintering furnace (301). The length of the one or more coils (302) below 300 millimeter refrains to obtain the optimized sintering process due to decrease in feed rate and beyond 400 millimeter increases maintenance cost of the one or more coils.
[0026] A coupler (501) (as shown is FIG. 5) is used to hold the soot blank (101) through the top conical zone (106) to help it suspending vertically into the sintering furnace (301) with the help of a handle (502). The coupler (501) is present inside the sintering tower. The coupler needs to be placed in such a way that it never comes close to the sintering furnace (301) as high temperature of the sintering furnace will deteriorate the coupler (501). Due to which, the top conical zone (106) does not undergo sintering so as to prevent the coupler (501) from

deteriorating during the variable sintering of the soot blank. The heating furnace (sintering furnace) (301) has the parabolic temperature profile on the basis of the one or more coils (302) and is parabolic in shape because as we move farther from the one or more coils, the temperature decreases. The temperature profile determines the length of the effective heating zone.
[0027] Each zone of the soot blank (101) is sequentially sintered in the effective heating zone (303). The sequential sintering implies sintering of the plurality of soot blank zones in a sequence i.e., the bottom conical zone (103) of the soot blank is sintered first, then a zone/portion above it and so on, excluding the top conical zone, at the predefined plurality of sintering temperatures and feed rates. Considering the seven zones in between the upper middle zone (105) and the lower middle zone (104), the bottom conical zone (103), the upper middle zone (105) and the lower middle zone (104) make a total of 10 zones in the soot blank (101) that undergo sintering at the predefined plurality of sintering temperatures and feed rates in the sintering furnace (301). For example, the soot blank (101) may have 10 zones of equal size. All the 10 zones of the soot blank (101) may be sintered at the predefined plurality of sintering temperatures and feed rates on the basis of radial density profile of the soot blank, diameter of the soot blank and temperature profile of the heating/sintering furnace. The sintering process for different weights of the soot blank exhibits different average diameter variation along a length of the sintered glass preform (402) due to difference in radial density profile of the sintered glass preform, which is one input parameter for optimization of the sintering process. In an example, the weight of the soot blank (101) in a range of about 40-60 kg may have a diameter variation of about <+2mm. In another example, the weight of the soot blank (101) in a range of about 30-40 kg may have along the length of the preform diameter variation of about <+lmm. Alternatively, the weight of the preform may vary. Since different weights of the preform have a difference in radial density profile that determines the feed rate and the sintering temperature, there is a difference in the diameter variation along the length of the preform.

[0028] Thus, the process of sintering is optimized by having the plurality of soot blank zones and each of the plurality of soot blank zones of the soot blank may have the predefined plurality of sintering temperatures and feed rates on the basis of radial density profile of the soot blank, diameter of the soot blank and temperature profile of the sintering furnace.
[0029] Length of each of the plurality of soot blank zones (103,
104 105, 106) may be less than a length (or the zone length or an effective
heating zone length) of the effective heating zone for optimal sintering. The effective heating zone (or heating zone) (303) (shown in FIG. 3) is an area, where the sintering process of the soot blank is possible. The effective heating zone length may be calculated on the basis of the temperature profile and has sufficient temperature for the sintering process. The effective heating zone length is comparatively larger than a length of the one or more coils (i.e., heating coils) (302), because the effective heating zone length is based on the temperature profile of the sintering furnace. The temperature profile of the sintering furnace is determined on the basis of the length of the one or more coils (302). The temperature profile is the parabolic temperature profile as the temperature decreases when we move farther from the one or more coils (302). The effective heating zone is up to the length, where the temperature at both ends of the parabolic temperature profile is 1400 degree Celsius as below this temperature, the sintering process of the soot blank cannot be done.
[0030] The effective heating zone is a single heating zone. To have better control on the sintering behaviour of glass, the soot blank is segregated into the plurality of soot blank zones. The length of each of the plurality of soot blank zones implies the length of a single zone of the plurality of soot blank zones and is measured on the basis of radial density profile, diameter of the soot blank and temperature of the sintering furnace. In order to form the sintered glass preform, the soot blank is vertically suspended such that the bottom conical zone of the soot blank is sintered first. The sintering temperature is at least 1520 degree Celsius because below this the soot blank results in partial sintering and cloudy patches.

[0031] Each of the plurality of soot blank zones (as shown in FIG. 2) may be shorter in length as compared to the total length of the soot blank (101). Further, each zone of the plurality of soot blank zones may have the predefined plurality of sintering temperatures and feed rates. The plurality of soot blank zones (103, 104,...., 105, 106) has a plurality of residence times in the effective heating zone (303). That is, a residence time for each zone of the plurality of soot blank zones may be different so as to avoid formation of viscous creep and over-stretching into the soot blank (101). The residence time is a ratio of length of the effective heating zone and feed rate. The residence time helps in determining time taken for each of the plurality of soot blank zones to reside in the effective heating zone for complete sintering. In an example, if the feed rate of the bottom conical zone (103) of the soot blank (101) is 3mm/minute and length of the effective heating zone is 500 mm, then the residence time will be 166.6 minute. In another example, if the feed rate of the bottom conical zone (103) of the soot blank (101) is 4.5 mm/min and length of the effective heating zone is 500mm, then the residence time will be 111.1 minute.
[0032] The predefined plurality of sintering temperatures and feed rates may be determined based on three input parameters i.e., radial density profile of the soot blank, the temperature profile of the sintering furnace and the diameter of the soot blank. The radial density of the soot blank (106) is different at every point along radius and is determined from centre to circumference. An average of the radial density provides diameter of the soot blank (101) and the diameter of the soot blank is important factor in determining the feed rate and temperature. The radial density profile impacts the time required by a particular cross section of the soot blank for the sintering. Further, the feed rate proportionality is calculated numerically with the help of glass viscosity of the soot blank and sintering time dependence on temperature.
[0033] As discussed previously, each of the plurality of soot blank zones has a length less than the length of the effective heating zone. Alternatively, the plurality of soot blank zones of the soot blank has different lengths on the basis of the parabolic temperature profile of the sintering furnace of radial density profile

of soot blank, and diameter of soot blank. The effective heating zone (303) may have a length as 500-600 millimeter in the sintering furnace, where below 500 millimeter, the feed rate decreases and sintering time increases and the sintering process may not be optimized and beyond 600 millimeter, the length of the one or more coils needs to be increased since the length of the effective heating zone is based on the length of the one or more coils (302), which is an expensive way to go since it may increase the maintenance cost of the one or more coils (302).
[0034] The soot blank may have a bulk density of about 0.35-0.45 g/cc as per the capacity of the sintering furnace. The feed rate depends upon the bulk density of the soot blank. If bulk density changes so does the feed rate of soot blank will change. Alternatively, the bulk density of the soot blank may vary. In an example, the soot blank may have the bulk density in a range of about 0.3-0.6 g/cc. In another example, the range of bulk density of the soot blank may vary because every soot blank is of different weight and size. The feed rate of the soot blank (101) may be in a range of about 3-8 mm/min. When the feed rate of the soot blank (101) is below 3 mm/min then sintering time process increases, while feed rate above 8 mm/min is employed for the preforms which are thin. The sintering furnace may have an optimum sintering temperature in a range of 1520-1600°C, which is determined on the basis of the temperature profile of the sintering furnace. Negative impact of sintering temperature below 1520 degree Celsius is either an increase in time taken for the sintering process or soot blank will not be sintered properly. Above 1600 degree Celsius, the sintering temperature may create an obstacle towards controlling diameter variation along the length of preform since it will affect feed rate of soot blank.
[0035] FIG. 4 illustrates an example of a diameter variation of the sintered glass preform (402) with respect to the soot blank (101) in accordance with the present disclosure. In an example, diameter (D) represented as (403) of the soot blank (101) is in a range of about 150-400 mm. A maximum diameter (Dl) represented as (404) and a minimum diameter (D2) represented as (405) along the length of the sintered glass preform (402) may be 133 millimeter and 130 millimeter respectively. In this case, the difference between the maximum

diameter and the minimum diameter is 3 millimeter and thus, the diameter variation falls well within the range of 1-4 millimeter. In another example, an average diameter of the sintered glass preform (402) may be 130 millimeter, where the diameter change will be within +1-2 millimeter, which implies that the diameter of the sintered glass preform may be 128 millimeter at some point of the sintered glass preform or may be 132 millimeter at some other point.
[0036] The sintered glass preform (402) is transparent with a low diameter variation and is a high-quality sintered glass preform (402) with a reduced cloudy region and top bend. Generally, different preforms will have different weights and thus, will have different radial density profile, the predefined plurality of sintering temperatures and feed rates. The sintered glass preform (402) upholds a perfect cylindrical geometry with the low diameter variation along the length of preform that is maintained at 1-4 mm and also maintains eccentricity which implies circularity of cylindrical soot preform will be maintained.
[0037] In conventional methods, as shown in FIG. 1, the diameter variation along the length of the sintered glass preform (102) was 7-8 mm that not only degrades physical quality of the sintered glass preform, but also reduces optical properties. The problem of high diameter variation i.e., in the range of 7-8 millimeter in the soot blank has not been solved in any of the prior-arts. Advantageously, the proposed variable sintering method has reduced the diameter variation along the length of the sintered glass preform to 1-4 mm and produces the sintered glass preform of superior physical quality that exhibits better optical properties as the diameter variation is low.
[0038] FIG. 6 is a flow chart (600) illustrating a method of variable sintering of the soot blank (101).
[0039] The soot blank is hung vertically over the effective heating zone (303) from where the soot blank (101) is lowered down into the effective heating zone (303) gradually. The variable sintering of the soot blank includes enabling a relative motion between the soot blank and the effective heating zone, since the soot blank undergoes sintering vertically with the help of the coupler (501). The relative motion between the soot blank (101) and the effective heating zone (303)

enables sequential sintering of the plurality of soot blank zones (103, 104 and 105). The soot blank is suspended such that the bottom conical zone (103) of the soot blank is sintered first. The bottom conical zone (103) of the soot blank is sintered at a higher value of the sintering temperature and the feed rate as compared to the upper middle zone (105) of the soot blank to avoid viscous creep into the soot blank. The bottom conical zone (103) of the soot blank is sintered at a higher value of feed rate compared to the upper middle zone (105) of the soot blank, where it is cylindrical in shape. That is, the upper middle zone of the soot blank is sintered at a lower value of temperature as compared to the bottom conical zone of the soot blank. Also, the upper middle zone (105) of the soot blank where it is cylindrical in shape is sintered at lower value of the feed rate as compared to the bottom conical zone of the soot blank.
[0040] Specifically, at step (601), the method includes suspending the soot blank (101) vertically in the sintering tower. The suspension of the soot blank is done through the coupler (501), whose function is to hold the soot blank through the top conical zone (106) and is present inside the sintering tower. The coupler (501) needs to be placed in such a way that it never comes close to the sintering furnace as the high temperature will deteriorate the coupler (501).
[0041] At step (602), the method includes heating the soot blank (101) in the sintering furnace of the sintering tower to initiate the process of sintering the bottom conical zone (103). The sintering of the bottom conical zone (103) is done in the effective heating zone (303). The effective heating zone is defined by a length. The length of the effective heating zone is based on the temperature profile of the sintering furnace which is parabolic in shape.
[0042] At step (603), the method include enabling the relative motion between the soot blank and the effective heating zone so that each of the plurality of soot blank zones can undergo sintering. Since the soot blank is suspended vertically, the relative motion will be vertically between the soot blank and the sintering furnace.

[0043] At step (604), the method includes the sequential sintering of the plurality of soot blank zones. The sequential sintering is due to the relative motion between the soot blank and the effective heating zone.
[0044] At step (605), the method includes sintering the bottom conical zone (103) of the plurality of soot blank zones of the soot blank. The feed rate will be high during sintering of the bottom conical zone (103) of the soot blank.
[0045] At step (606), the method includes sintering the zone/part just above the bottom conical zone and eventually remaining zones of the plurality of soot blank zones will get sintered one by one at same time at the predefined plurality of sintering temperatures and feed rates due to difference in the radial density profile.
[0046] Lastly, at step (607), the method includes sintering a top zone of the soot blank, where it is cylindrical in shape. The top zone is the upper middle zone (105), which further implies exclusion of the top conical zone (106) to prevent the coupler from deteriorating during sintering. The feed rate and the sintering temperature of the upper middle zone of the soot blank will be lower than the feed rate and the sintering temperature of the bottom conical zone so as to prevent overstretching and formation of viscous creep into the soot blank. The remaining zones of the plurality of soot blank zones are sintered at the predefined plurality of sintering temperatures and feed rates.
[0047] An overall sintering time for different weights of the soot blank changes due to difference in weight and time taken for sintering.
[0048] Advantageously, the proposed method of sintering the soot blank eliminates diameter variation during sintering without employing stretching process. Stretching process is employed for glass preform when they are taken to a draw tower to stretch it down to manufacture optical fiber. In the prior arts, the diameter variation is measured and controlled for the preform when it is taken to the draw tower to obtain optical fiber. The proposed method of variable sintering of the soot blank forms the sintered glass preform with a superior physical quality. Further, the method of variable sintering of the soot blank employs for any weight of the soot blank. Furthermore, the method of sintering of the soot blank can be

optimised for each of the soot blank. Optimisation for each of the soot blank may vary as per weight of the soot blank, length of the soot blank, radial density of the soot blank, diameter of the soot blank, temperature of sintering furnace.
[0049] The various actions, acts, blocks, steps, or the like in the flow chart may be performed in the order presented, in a different order or simultaneously. Further, in some implementations, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the invention.
[0050] Conditional language used herein, such as, among others, "can," "may," "might," "may," "e.g.," and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain alternatives include, while other alternatives do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more alternatives or that one or more alternatives necessarily include logic for deciding, with or without other input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular alternative. The terms "comprising," "including," "having," and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term "or" is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term "or" means one, some, or all of the elements in the list.
[0051] Disjunctive language such as the phrase "at least one of X, Y, Z," unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (e.g., X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain alternatives require at least one of X, at least one of Y, or at least one of Z to each be present.
[0052] While the detailed description has shown, described, and pointed out novel features as applied to various alternatives, it can be understood that

various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the scope of the disclosure. As can be recognized, certain alternatives described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others.

CLAIMS
We Claim:

1. A method of variable sintering of a soot blank (101), the method
comprising:
heating the soot blank (101) in an effective heating zone (303), wherein the soot blank has a plurality of soot blank zones (103, 104, 105, 106) along a longitudinal axis (107) of the soot blank (101), wherein the plurality of soot blank zones (103, 104, 105) is heated at a predefined plurality of sintering temperatures to form a sintered glass preform (402), wherein the sintered glass preform (402) has a diameter variation in a range of 1-4 mm measured along a length of the sintered glass preform (402).
2. The method as claimed in claim 1, wherein each of the plurality of soot blank zones is of equal length.
3. The method as claimed in claim 1, wherein the plurality of soot blank zones (103, 104, 105) is fed at a plurality of feed rates in the effective heating zone (303).
4. The method as claimed in claim 1, wherein the plurality of soot blank zones (103, 104, 105) has a plurality of residence times in the effective heating zone (303).
5. The method as claimed in claim 1 further comprising determining a sintering temperature and a feed rate of each of the plurality of soot blank zones (103, 104, 105) based on a radial density profile of the soot blank, a temperature profile of a sintering furnace and a diameter of the soot blank.
6. The method as claimed in claim 1, further comprising:

enabling a relative motion between the soot blank (101) and the effective heating zone (303), wherein the relative motion between the soot blank (101) and the effective heating zone (303) enables sequential sintering of the plurality of soot blank zones.
7. The method as claimed in claim 1, wherein the effective heating zone (303) has a zone length less than a length of the soot blank (101).
8. The method as claimed in claim 1, wherein the effective heating zone (303) is a region where temperature is sufficient for sintering and is determined by temperature profile which is based on a coil length.
9. The method as claimed in claim 1, wherein each of the plurality of soot blank zones has a length less than the zone length of the effective heating zone.
10. The method as claimed in claim 1, wherein the soot blank (101) has a top conical zone (106) and a bottom conical zone (103).
11. A sintering furnace (301), comprising:
an effective heating zone (303), wherein the effective heating zone has a zone length less than a length of a soot blank (101), wherein the effective heating zone (303) has one or more coils (302), wherein the one or more coils (302) has a coil length greater than or equal to 300 mm to create the effective heating zone (303) for variable sintering; and
a coupler (501) for suspending the soot blank (101) vertically into the sintering furnace (301),
wherein the soot blank (101) has a plurality of soot blank zones (103, 104, 105, 106) along a longitudinal axis (107) of the soot blank (101), wherein the plurality of soot blank zones (103, 104, 105) of the soot blank (101) is heated at a predefined plurality of sintering temperatures to

form a sintered glass preform (402), wherein the sintered glass preform (402) has a diameter variation in a range of 1-4 mm measured along a length of the sintered glass preform (402).
12. The sintering furnace (301) as claimed in claim 11, wherein the sintering furnace is defined by a parabolic temperature profile for determining an optimum sintering temperature and feed rate for each of the plurality of soot blank zones.
13. The sintering furnace (301) as claimed in claim 11, wherein the sintering furnace has a sintering temperature of at least 1520°C.