Title of Invention	METHOD AND APPARATUS FOR MAKING STRAIGHT CORE ROD FOR MANUFACTURING OPTICAL FIBER
Abstract	The present invention relates to a method and apparatus for preparing straight core rod. The method comprises monitoring the wobbling of the part of the substrate tube that is in the hot zone along with simultaneous changing the rotation speed of the substrate tube based on the degree of bend observed. The apparatus includes a charge coupled device (CCD) camera with inbuilt software that is used to monitor and quantify the bend and a PLC that responds to the quantified bend in the substrate tube to straighten it by changing the speed of rotation of the substrate tube based on its angular position (offset A) during collapsing process step. According to present invention, the apparatus is provided for simultaneous monitoring and controlling the straightness of the substrate tube during its collapsing stage.

Title of Invention

METHOD AND APPARATUS FOR MAKING STRAIGHT CORE ROD FOR MANUFACTURING OPTICAL FIBER

Abstract

The present invention relates to a method and apparatus for preparing straight core rod. The method comprises monitoring the wobbling of the part of the substrate tube that is in the hot zone along with simultaneous changing the rotation speed of the substrate tube based on the degree of bend observed. The apparatus includes a charge coupled device (CCD) camera with inbuilt software that is used to monitor and quantify the bend and a PLC that responds to the quantified bend in the substrate tube to straighten it by changing the speed of rotation of the substrate tube based on its angular position (offset A) during collapsing process step. According to present invention, the apparatus is provided for simultaneous monitoring and controlling the straightness of the substrate tube during its collapsing stage.

Full Text	FORM 2 THE PATENTS ACT 1970 [39 OF 1970] PROVISIONAL SPECIFICATION [See Section 10] "METHOD FOR MAKING STRAIGHT CORE ROD FOR MANUFACTURING OPTICAL FIBER" STERLITE OPTICAL TECHNOLOGIES LIMITED, of E-2, MIDC, Waluj, Aurangabad 431136, Maharastra, India, The following specification describes the nature of the invention. 5 TITLE OF INVENTION Method for making straight core rod for manufacturing optical fiber BACKGROUND OF INVENTION: 1. Field of invention 10 Optical fiber used in the communication systems and operating in the visible and near visible spectrum of electromagnetic radiation has revolutionized the field of data transmission for their high data carrying capacity. The mostly widely used optical fibers today are manufactured from 15 silica based materials, with the fiber core region suitably doped with refractive index modifying materials. Different competing methods employed in the manufacture of silica based optical fiber is described in "Optical fiber communication" and the article entitled "Fiber optics" in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol 10, pp. 125-147 (1994) 20 and both are incorporated herein. All these methods aims at preparing a glass preform as the first step in the manufacturing of the optical fiber. This is carried out by several competing methods like modified chemical vapor deposition (MCVD), outer vapor deposition (OVD) and vapor axial deposition (VAD). An optical fiber preform is basically an elongated cylindrical body 25 made up off silica based materials with the center region suitably doped with refractive index modifying materials. The subsequent common step in the drawing of optical fiber from these preform into a long thin fiber. Preparation of optical fiber following modified chemical vapor deposition 30 (MCVD) method involves the initial step of preparation of primary preform (also called core rod) following method originally described in the patent U.S. Pat. No. 4,217,027. The process involves layer after layer deposition of glassy material on the inside surface of a glass tube called the substrate tube. The deposited glassy materials mainly consist of pure silica and a small amount 5 of the dopant (generally germanium) to modify the refractive index of the glass for it to act as light guiding medium. After the deposition of desired layer of the glassy materials the tube is made to collapse by heating the substrate tube with a traversing heat source along its length. 10 2. Description of prior art; The optical fiber has a specific dimension that needs to be maintained throughout its length and the dimension of the optical fiber is proportional to that of the preform from which it is drawn. Thus not only the dimension of 15 the core rod but also the dimensional variation along the length of the core rod plays a vital role in dimensional consistency of the fiber thus drawn from it. Dimensional inconsistency of the core rod includes the ovality and non-straightness of the core rod, be it localized or throughout its length, can make the core rod being rejected when not meeting manufacturing standard. Non-20 straightness or bending of core rods is a serious problem and constitutes up to 6 % of the total physical failures of the core rod and with more than 50% of the total core rods produced are found to be have bend. The bend in the core rod occurs during deposition and collapsing stage of its preparation due to localized heating of the precursor substrate tube to high temperature for 25 longer duration. Prior art methods for preparation of straight core rod is described in U.S. Pat. No. 4,477,273 and 6,105,396. The use of carbon roller to straighten the substrate tube is disclosed in the U.S. Pat. No. 4,477,273. The carbon 30 roller engages a portion of the substrate tube that gets exposed to the zone of heat and reconfigure any offset in the substrate tube. Direct contact with the carbon roller causes particles on the surface of core rod and scratches over the core rod. This particle impinges inside the core rod during collapsing step and it will lead to break in optical fiber during optical fiber drawing. 5 Scratches on the core rod observed cause non-uniform collapsing inside second tube, while placing the substantial portion of the core rod inside the second tube and exposing the second tube with higher temperature to collapse the second tube with the core rod to form optical fiber preform. 10 U.S. Pat. No. 6,105,396 describes a method of using a machine-vision system that monitors the straightness of the tube and control it by varying the rotational speed according to its angular position. The machine-vision system described in the patent comprises of a laser source and a detector that are positioned on opposite sides of the substrate tube and "positioned to 15 monitor the portion of the tube, which has just been heated". The detector, which consists of a linear array of photodiodes that detect the shadow cast by the tube and for a tube that is not perfectly straight, the shadow of the substrate tube on the detector wobbles up and down. The controller responds to the wobbling and varies the rotational speed of the tube according to its 20 angular position. The laser source and the detector are placed behind the traversing torch. The non-straightness (called as bend) of the core rod occurs due to improper axial alignment between the chucks and non-uniform heating 25 source. The localized bend in the core rod has been observed to have more at the heating portion (hot zone) of the substrate tube One of the major problems associated with such prior art assembly setup is the inability to distinguish between a localized bend in the core rod and the bend arising out of cantilever effect during its manufacture. 30 The machine-vision system, i.e., laser source and detector are positioned between the torch assembly and the headstock chuck during deposition and between torch assembly and the tailstock chuck during collapsing step. Person familiar with this art will be aware of the fact that 5 presence of localized bend in the substrate tube cannot be made straight using such prior art assembly setup and correction of localized bend in substrate tube demands the presence of the sensor in the close proximity of the hot zone. With a localized bend present in the hot zone and the straight portion near the sensor as the sensor is far away from the heating portion 10 and no wobbling will be sensed by the detector and hence machine will not carry out any correction. And when the localized bend portion is near the sensor it will create high wobbling and the machine will respond to straighten the portion of substrate that doesn't need straightening and end up in creating more bend in the substrate tube due to inhomogeneous heating. 15 The present inventors observed that the bend in the tube/core rod increases during the collapsing step than that of the deposition step due to higher temperature in the tube during collapsing step than the deposition step of preparing core rod. Another disadvantage of the prior art method US 20 '396 is that it controls the straightness of the tube during only the deposition step and it has been observed that the bend in the core rod remains after collapsing of the process step. It is an observation that the prior art method does not provide the straight core rods after collapsing step. 25 The present inventor observed that such problem could be overcome only when the wobbling monitoring unit is in close proximity of the hot zone and better result can be obtained when it is in the hot zone itself and also controlling the straightness of the core rod during collapsing step. 30 3. Need of the invention Therefore, there is a need to have a method and apparatus for preparing straight core rod by controlling the straightness of the core rod during collapsing step and to monitoring the straightness of the core rod in 5 close proximity of the hot zone of the substrate tube and overcoming the above-described problems of the prior art. 4. Objects of the invention 10 The main object of the present invention is a method and apparatus for preparing straight core rod by collapsing a substrate tube and keeping substrate tube straight, during collapsing the substrate tube. Another object of the present invention is to provide the apparatus to 15 monitor and detect the straightness (bending) of the tube in close proximity of the hot zone of the substrate tube. Still another of the object of the present invention is to provide a method and apparatus to control the bending of the substrate tube during 20 collapse step. In preferred embodiment of the present invention, the method for preparing straight core rod comprises monitoring the wobbling of the part of the substrate tube that is in the hot zone along with simultaneous changing 25 the rotation speed of the substrate tube based on the degree of bend (offset ∆) observed. The apparatus includes a charge coupled device (CCD) camera with inbuilt software that is used to monitor and quantify the bend and a PLC that responds to the quantified bend in the substrate tube to straighten it by changing the speed of rotation of the substrate tube based on its angular 30 position [offset A] during collapsing process step. 5 BRIEF DESCRIPTION OF THE FIGURES The invention and its mode of operation will be more clearly understood from the following detailed description when read with the accompanying drawing in which: 10 Fig. 1 is a schematic representation of the glass working lathe on which the collapsing of the substrate tube takes place with apparatus of the present invention attached. Fig. 2 is a schematic representation of the field of view of the CCD camera 15 presented on a cartesian plane. Fig. 3 is a plot of the intensity variation as a function of radial position of substrate tube. Fig. 4 is a plot of the change in light intensity along the radial position of substrate tube. 20 DETAIL DESCRIPTION OF THE PREFERRED INVENTION Preparation of optical fiber following modified chemical vapor deposition (MCVD) method involves the initial step of preparation of primary preform 25 (also called as core rod 100) following method described in the prior art method. The deposition of clad and core material inside the substrate tube is well known in the prior art. The process of the collapsing of a substrate tube is schematically presented in figure 1. The substrate tube 101 is held between two chucks of a glass working lathe (not shown) and rotated about 30 its longitudinal axis. A flame torch 103, being mounted on a movable bench 102, is placed underneath the substrate tube 101 so as to heat the latter. Also mounted on the movable bench 102 is a charge coupled device (CCD) camera 104. The mounting of the torch 103 and the CCD camera 104 ensures the movement of both these entities to traverse at constant speed. The CCD camera 104 has an inbuilt computer with special software that is used to monitor and quantify the bend and a Programmable Logic Controller [PLC] that responds to the quantified bend in the substrate tube to straighten it by changing the speed of rotation of the substrate tube based on 10 its angular position during collapsing process step. The apparatus of the present invention consists of a monitoring unit and a controlling unit. The monitoring unit includes a charge coupled device (CCD) camera 104 with inbuilt computer with a special software loaded 15 software that enables the monitoring unit to not only monitor but also compute and express the degree of bend in digital format for the PLC 105a recognize and respond according to a particular logic. The monitoring unit is used to determine the bend in the core rod and quantify the extent of bent as offset (∆). The value of the offset (∆) is provided to the PLC. The PLC responds 20 to the value of offset (∆) by changing the substrate tube rotation speed according to the expression as follows:  = S + g. ∆ 25 Where  = rotation speed set by the PLC; S = set rotation speed in the recipe; g = gain ( a constant); and ∆ = offset. 30 The value of the offset (∆) is monitored every 20 millisecond and fed to the PLC which responds to the value by changing the speed of rotation of the core rod. The working of the monitoring unit can be described using figure 2-4. The figure 2 is the schematic representation of the field of view of the CCD camera 104 with the bright area representing the image of the portion of the substrate tube 104b in the hot zone as seen against a dark background 10 105b. The field of view of the CCD camera 104 being two-dimensional acts as cartesian x-y plane. The inbuilt software can assign coordinate values to any point on the image. In this regard an array of photosites 108 is selected. The intensity of the light on each photosites of the array 108 is analyzed. A plot of light intensity as sensed by the photosites in the selected array 108 is plotted 15 against their position along the y-direction as shown in figure 3. The figure 3 is characterized by a low intensity region 111 and 112 and an high intensity region 113, with regions 106 and 107 showing a sudden change in the light intensity. The low intensity region 111 and 112 representing dark back ground whereas the high intensity region 113 representing the substrate tube 20 104 in the hot zone. The region of sudden change in the intensity of radiation 106 and 107 represents the bottom and top edge of the substrate tube 101 respectively. In order to assign the coordinate value to the substrate tube edges a 25 plot of change in radiation intensity with change in radial position along the y-direction is plotted against the radial position of the substrate tube 101 (figure 4). In this figure two points of highest gradient intensity 106 and 107 are observed. The radial position corresponding to the highest gradient intensity 109 and 110 represents the edge of the substrate tube. The next 30 step involves the data processing for obtaining the value of offset (A). The mid point of the substrate tube is determined by averaging the edge 109 and edge 110. At any given time one mid point value of the substrate tube is obtained. This is compared with the average value of 5 previously recorded set of data collected to the present value to give the value of offset (A). For example if the value of the mid point as recorded at time t1, t2, t3, ,t10 are P1, P2, P3, , P10 respectively and the data collected at time t11 is Pn then offset at time tn, (∆11) is defined as 10 An = [(P1 + P2 + P3 + + P10)/10] - P11 and similarly the offset measured at time t12 is ∆12 = [(P2 + P3 + P4 + + P11)/10] - P12 The value of offset (∆) can be both positive and negative. The positive value of 15 the offset means the substrate tube is bend towards downward and negative value of offset indicates the substrate tube bend towards upward. Thus the positive value of offset will lead to an increase in the rotation speed whereas negative value of offset will decrease in the rotation speed. Decrease in the rotation speed as the offset value acquires a negative value and with the 20 substrate tube bending downward, means the bottom edge of the substrate tube will be exposed to heat source for longer duration leading to the straightening of the substrate tube by virtue of gravity. Example: 25 For a substrate tube that is rotating at a set rotation speed S of 16 and observed offset value is 30 the rotation speed set by the PLC becomes 23 when the gain is set at a typical value of 0.25. 30 References Cited: US patent No: 6,105,396 22/08/2000 Glodis et al., U.S. Pat. No. 4,217,027 12/08/1980 MacChesney et al., U.S. Pat. No. 4,477,273 16/10/1984 Lynch et al., 5 Other Publications Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 10, pp. 125-147 (1994) Optical Fiber Communications vol. 1, Fiber Fabrication, Academic Press Inc., 1985, pp 1-177 10 5 We claim: 1. A method for preparing straight core rod [100] for manufacturing optical fiber comprising the steps of: a. depositing clad and core material inside the substrate tube [101]; 10 and b. collapsing the deposited substrate tube to form core rod [100]; wherein said collapsing step is characterized by, straightening the core rod [100] during said collapsing step. 15 2. The method as claimed in claim 1, wherein the straightening of the core rod [100] comprising the steps of: a. monitoring hotzone portion [104b/105b] of the substrate tube [101] in close proximity of the hotzone portion [104b/105b] of the tube using a monitoring unit [104]; 20 b. determining the bend (offset ∆) of the substrate tube [101] from a set of mid points of said substrate tube [101]; and c. changing the rotation speed () of the substrate tube [101] based on bend (offset ∆) of said substrate tube [101]. 25 3. The method as claimed in claim 2, wherein said monitoring unit is a Charged Coupled Device (CCD) Camera [104] with inbuilt software to determine the position of mid point of said substrate tube in the hot zone. 30 4. The method as claimed in claim 2, wherein said monitoring unit [104] is placed in close proximity of the hotzone [104b/ 105b] of the substrate tube. 5 5. The method as claimed in claim 2, wherein determining the bend (offset A) of the substrate tube comprises the steps of: a. determining the edges [109/110] of the substrate tube [101] from the monitoring unit [104]; b. identifying the mid point of substrate tube by averaging both end 10 of edges [109/110] of the substrate tube [101]; c. obtaining mid point of substrate tube [101] continuously at any given time; and d. comparing the average value of previously recorded set of mid point to the present value of mid point of substrate tube. 15 6. The method as claimed in claim 2, wherein the rotation speed () of the substrate tube [101] is changed using the following equation:  = S + g . ∆ wherein,  = actual rotation speed set by the PLC [105] during 20 collapsing step; S - set rotation speed in the recipe; g = gain ( a constant); and ∆ = offset (bend). 25 7. A method for preparing straight core rod [100] for manufacturing optical fiber comprising the steps of: a. depositing clad and core material inside the substrate tube [101]; b. collapsing the deposited substrate tube to form core rod [100]; and 30 c. straightening the core rod [100] during the collapsing step, wherein, said straightening step c) characterized by the following steps: 5 i. monitoring hotzone portion [104b/105b] of the substrate tube in close proximity of the hotzone portion [104b/105b] of the tube using monitoring unit [104]; ii. determining the bend (offset ∆) of the substrate tube [101] from the set of mid points of said substrate tube [101]; and 10 iii. changing the rotation speed () of the substrate tube [101] based on bend (offset ∆) of said substrate tube [101]. 8. The method as claimed in claim 7, wherein said monitoring unit is the Charged Coupled Device (CCD) Camera [104] with inbuilt software to 15 determine the position of mid point of said substrate tube in the hot zone. 9. The method as claimed in claim 7, wherein said monitoring unit [104] is placed close proximity of the hotzone [104b/105b] of the substrate 20 tube. 10. The method as claimed in claim 7, wherein determining the bend (offset ∆) of the substrate tube comprises the steps of: a. determining the edges [109/110] of the substrate tube [101] from 25 the monitoring unit [104]; b. identifying the mid point of substrate tube by averaging both end of edges [109/110] of the substrate tube [101]; c. obtaining mid point of substrate tube [101] continuously at any given time; and 30 d. comparing the average value of previously recorded set of mid point to the present value of mid point of substrate tube. 11. The method as claimed in claim 7, wherein the rotation speed () of the substrate tube [101] is changed using the following equation:  = S + g. ∆ wherein,  = actual rotation speed set by the PLC [105] during collapsing step; S = set rotation speed in the recipe; g = gain ( a constant); and ∆ = offset (bend). 12. An apparatus for producing straight core rod [100], wherein said apparatus comprises of: a. monitoring unit [104] positioned in close proximity of the hotzone [104b/105b] of the substrate tube [101]; and b. controlling unit [105] to control the rotation speed of the substrate tube [101] based on its angular position (offset A); 13. An apparatus as claimed in claim 12, wherein said monitoring unit is charged couple device (CCD) camera [104] with inbuilt software to determine the position of mid point of said substrate tube in the hot zone. 14. An apparatus as claimed in claim 12, wherein said monitoring unit [104] is provided to monitor the hotzone of the substrate tube, identify the edges [109/110] and mid point of substrate tube and to calculate the bend (offset A) of the substrate tube by comparing the average value of previously collected set of mid point to the present value of mid point of the substrate tube. 15. An apparatus as claimed in claim 12, wherein said controlling unit is PLC controller [105] to control the rotation speed of the substrate tube [101] based on the offset A obtained by the calculating unit. 16. A core rod [100] produced in accordance with any one of the preceding claims. 17. An optical fiber preform produced in accordance with any of claims 1-15. 18. An optical fiber produced in accordance with any of claims 1-15.

Full Text

FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
PROVISIONAL SPECIFICATION
[See Section 10]
"METHOD FOR MAKING STRAIGHT CORE ROD FOR MANUFACTURING OPTICAL FIBER"
STERLITE OPTICAL TECHNOLOGIES LIMITED, of E-2, MIDC, Waluj, Aurangabad 431136, Maharastra, India,
The following specification describes the nature of the invention.

5 TITLE OF INVENTION
Method for making straight core rod for manufacturing optical fiber
BACKGROUND OF INVENTION: 1. Field of invention
10
Optical fiber used in the communication systems and operating in the visible and near visible spectrum of electromagnetic radiation has revolutionized the field of data transmission for their high data carrying capacity. The mostly widely used optical fibers today are manufactured from
15 silica based materials, with the fiber core region suitably doped with refractive index modifying materials. Different competing methods employed in the manufacture of silica based optical fiber is described in "Optical fiber communication" and the article entitled "Fiber optics" in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol 10, pp. 125-147 (1994)
20 and both are incorporated herein. All these methods aims at preparing a glass preform as the first step in the manufacturing of the optical fiber. This is carried out by several competing methods like modified chemical vapor deposition (MCVD), outer vapor deposition (OVD) and vapor axial deposition (VAD). An optical fiber preform is basically an elongated cylindrical body
25 made up off silica based materials with the center region suitably doped with refractive index modifying materials. The subsequent common step in the drawing of optical fiber from these preform into a long thin fiber.
Preparation of optical fiber following modified chemical vapor deposition 30 (MCVD) method involves the initial step of preparation of primary preform (also called core rod) following method originally described in the patent U.S. Pat. No. 4,217,027. The process involves layer after layer deposition of glassy material on the inside surface of a glass tube called the substrate tube. The deposited glassy materials mainly consist of pure silica and a small amount

5 of the dopant (generally germanium) to modify the refractive index of the glass for it to act as light guiding medium. After the deposition of desired layer of the glassy materials the tube is made to collapse by heating the substrate tube with a traversing heat source along its length.
10 2. Description of prior art;
The optical fiber has a specific dimension that needs to be maintained throughout its length and the dimension of the optical fiber is proportional to that of the preform from which it is drawn. Thus not only the dimension of 15 the core rod but also the dimensional variation along the length of the core rod plays a vital role in dimensional consistency of the fiber thus drawn from it. Dimensional inconsistency of the core rod includes the ovality and non-straightness of the core rod, be it localized or throughout its length, can make the core rod being rejected when not meeting manufacturing standard. Non-20 straightness or bending of core rods is a serious problem and constitutes up to 6 % of the total physical failures of the core rod and with more than 50% of the total core rods produced are found to be have bend. The bend in the core rod occurs during deposition and collapsing stage of its preparation due to localized heating of the precursor substrate tube to high temperature for 25 longer duration.
Prior art methods for preparation of straight core rod is described in U.S. Pat. No. 4,477,273 and 6,105,396. The use of carbon roller to straighten the substrate tube is disclosed in the U.S. Pat. No. 4,477,273. The carbon 30 roller engages a portion of the substrate tube that gets exposed to the zone of heat and reconfigure any offset in the substrate tube. Direct contact with the carbon roller causes particles on the surface of core rod and scratches over the core rod. This particle impinges inside the core rod during collapsing step and it will lead to break in optical fiber during optical fiber drawing.

5 Scratches on the core rod observed cause non-uniform collapsing inside second tube, while placing the substantial portion of the core rod inside the second tube and exposing the second tube with higher temperature to collapse the second tube with the core rod to form optical fiber preform.
10 U.S. Pat. No. 6,105,396 describes a method of using a machine-vision
system that monitors the straightness of the tube and control it by varying the rotational speed according to its angular position. The machine-vision system described in the patent comprises of a laser source and a detector that are positioned on opposite sides of the substrate tube and "positioned to
15 monitor the portion of the tube, which has just been heated". The detector, which consists of a linear array of photodiodes that detect the shadow cast by the tube and for a tube that is not perfectly straight, the shadow of the substrate tube on the detector wobbles up and down. The controller responds to the wobbling and varies the rotational speed of the tube according to its
20 angular position. The laser source and the detector are placed behind the traversing torch.
The non-straightness (called as bend) of the core rod occurs due to improper axial alignment between the chucks and non-uniform heating
25 source. The localized bend in the core rod has been observed to have more at the heating portion (hot zone) of the substrate tube One of the major problems associated with such prior art assembly setup is the inability to distinguish between a localized bend in the core rod and the bend arising out of cantilever effect during its manufacture.
30
The machine-vision system, i.e., laser source and detector are positioned between the torch assembly and the headstock chuck during deposition and between torch assembly and the tailstock chuck during collapsing step. Person familiar with this art will be aware of the fact that

5 presence of localized bend in the substrate tube cannot be made straight using such prior art assembly setup and correction of localized bend in substrate tube demands the presence of the sensor in the close proximity of the hot zone. With a localized bend present in the hot zone and the straight portion near the sensor as the sensor is far away from the heating portion
10 and no wobbling will be sensed by the detector and hence machine will not carry out any correction. And when the localized bend portion is near the sensor it will create high wobbling and the machine will respond to straighten the portion of substrate that doesn't need straightening and end up in creating more bend in the substrate tube due to inhomogeneous heating.
15
The present inventors observed that the bend in the tube/core rod increases during the collapsing step than that of the deposition step due to higher temperature in the tube during collapsing step than the deposition step of preparing core rod. Another disadvantage of the prior art method US
20 '396 is that it controls the straightness of the tube during only the deposition step and it has been observed that the bend in the core rod remains after collapsing of the process step. It is an observation that the prior art method does not provide the straight core rods after collapsing step.
25 The present inventor observed that such problem could be overcome
only when the wobbling monitoring unit is in close proximity of the hot zone and better result can be obtained when it is in the hot zone itself and also controlling the straightness of the core rod during collapsing step.
30 3. Need of the invention
Therefore, there is a need to have a method and apparatus for preparing straight core rod by controlling the straightness of the core rod during collapsing step and to monitoring the straightness of the core rod in

5 close proximity of the hot zone of the substrate tube and overcoming the above-described problems of the prior art.
4. Objects of the invention
10 The main object of the present invention is a method and apparatus for
preparing straight core rod by collapsing a substrate tube and keeping substrate tube straight, during collapsing the substrate tube.
Another object of the present invention is to provide the apparatus to 15 monitor and detect the straightness (bending) of the tube in close proximity of the hot zone of the substrate tube.
Still another of the object of the present invention is to provide a method and apparatus to control the bending of the substrate tube during 20 collapse step.
In preferred embodiment of the present invention, the method for preparing straight core rod comprises monitoring the wobbling of the part of the substrate tube that is in the hot zone along with simultaneous changing
25 the rotation speed of the substrate tube based on the degree of bend (offset ∆) observed. The apparatus includes a charge coupled device (CCD) camera with inbuilt software that is used to monitor and quantify the bend and a PLC that responds to the quantified bend in the substrate tube to straighten it by changing the speed of rotation of the substrate tube based on its angular
30 position [offset A] during collapsing process step.

5 BRIEF DESCRIPTION OF THE FIGURES
The invention and its mode of operation will be more clearly understood from the following detailed description when read with the accompanying drawing in which: 10
Fig. 1 is a schematic representation of the glass working lathe on which the collapsing of the substrate tube takes place with apparatus of the present invention attached.
Fig. 2 is a schematic representation of the field of view of the CCD camera 15 presented on a cartesian plane.
Fig. 3 is a plot of the intensity variation as a function of radial position of substrate tube.
Fig. 4 is a plot of the change in light intensity along the radial position of substrate tube. 20
DETAIL DESCRIPTION OF THE PREFERRED INVENTION
Preparation of optical fiber following modified chemical vapor deposition (MCVD) method involves the initial step of preparation of primary preform
25 (also called as core rod 100) following method described in the prior art method. The deposition of clad and core material inside the substrate tube is well known in the prior art. The process of the collapsing of a substrate tube is schematically presented in figure 1. The substrate tube 101 is held between two chucks of a glass working lathe (not shown) and rotated about
30 its longitudinal axis. A flame torch 103, being mounted on a movable bench 102, is placed underneath the substrate tube 101 so as to heat the latter. Also mounted on the movable bench 102 is a charge coupled device (CCD) camera 104. The mounting of the torch 103 and the CCD camera 104 ensures the movement of both these entities to traverse at constant speed.

The CCD camera 104 has an inbuilt computer with special software
that is used to monitor and quantify the bend and a Programmable Logic
Controller [PLC] that responds to the quantified bend in the substrate tube to
straighten it by changing the speed of rotation of the substrate tube based on
10 its angular position during collapsing process step.
The apparatus of the present invention consists of a monitoring unit and a controlling unit. The monitoring unit includes a charge coupled device (CCD) camera 104 with inbuilt computer with a special software loaded
15 software that enables the monitoring unit to not only monitor but also compute and express the degree of bend in digital format for the PLC 105a recognize and respond according to a particular logic. The monitoring unit is used to determine the bend in the core rod and quantify the extent of bent as offset (∆). The value of the offset (∆) is provided to the PLC. The PLC responds
20 to the value of offset (∆) by changing the substrate tube rotation speed according to the expression as follows:
 = S + g. ∆
25 Where
 = rotation speed set by the PLC;
S = set rotation speed in the recipe;
g = gain ( a constant); and
∆ = offset. 30
The value of the offset (∆) is monitored every 20 millisecond and fed to the
PLC which responds to the value by changing the speed of rotation of the core
rod.

The working of the monitoring unit can be described using figure 2-4. The figure 2 is the schematic representation of the field of view of the CCD camera 104 with the bright area representing the image of the portion of the substrate tube 104b in the hot zone as seen against a dark background
10 105b. The field of view of the CCD camera 104 being two-dimensional acts as cartesian x-y plane. The inbuilt software can assign coordinate values to any point on the image. In this regard an array of photosites 108 is selected. The intensity of the light on each photosites of the array 108 is analyzed. A plot of light intensity as sensed by the photosites in the selected array 108 is plotted
15 against their position along the y-direction as shown in figure 3. The figure 3 is characterized by a low intensity region 111 and 112 and an high intensity region 113, with regions 106 and 107 showing a sudden change in the light intensity. The low intensity region 111 and 112 representing dark back ground whereas the high intensity region 113 representing the substrate tube
20 104 in the hot zone. The region of sudden change in the intensity of radiation 106 and 107 represents the bottom and top edge of the substrate tube 101 respectively.
In order to assign the coordinate value to the substrate tube edges a 25 plot of change in radiation intensity with change in radial position along the y-direction is plotted against the radial position of the substrate tube 101 (figure 4). In this figure two points of highest gradient intensity 106 and 107 are observed. The radial position corresponding to the highest gradient intensity 109 and 110 represents the edge of the substrate tube. The next 30 step involves the data processing for obtaining the value of offset (A).
The mid point of the substrate tube is determined by averaging the edge 109 and edge 110. At any given time one mid point value of the substrate tube is obtained. This is compared with the average value of

5 previously recorded set of data collected to the present value to give the value of offset (A). For example if the value of the mid point as recorded at time t1,
t2, t3, ,t10 are P1, P2, P3, , P10 respectively and the data collected at
time t11 is Pn then offset at time tn, (∆11) is defined as
10 An = [(P1 + P2 + P3 + + P10)/10] - P11
and similarly the offset measured at time t12 is
∆12 = [(P2 + P3 + P4 + + P11)/10] - P12
The value of offset (∆) can be both positive and negative. The positive value of 15 the offset means the substrate tube is bend towards downward and negative value of offset indicates the substrate tube bend towards upward. Thus the positive value of offset will lead to an increase in the rotation speed whereas negative value of offset will decrease in the rotation speed. Decrease in the rotation speed as the offset value acquires a negative value and with the 20 substrate tube bending downward, means the bottom edge of the substrate tube will be exposed to heat source for longer duration leading to the straightening of the substrate tube by virtue of gravity.
Example:
25
For a substrate tube that is rotating at a set rotation speed S of 16 and
observed offset value is 30 the rotation speed set by the PLC becomes 23
when the gain is set at a typical value of 0.25.
30 References Cited:
US patent No: 6,105,396 22/08/2000 Glodis et al.,
U.S. Pat. No. 4,217,027 12/08/1980 MacChesney et al.,
U.S. Pat. No. 4,477,273 16/10/1984 Lynch et al.,

5 Other Publications
Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 10, pp. 125-147 (1994)
Optical Fiber Communications vol. 1, Fiber Fabrication, Academic Press Inc., 1985, pp 1-177 10

5 We claim:
1. A method for preparing straight core rod [100] for manufacturing optical fiber comprising the steps of:
a. depositing clad and core material inside the substrate tube [101];
10 and
b. collapsing the deposited substrate tube to form core rod [100];
wherein said collapsing step is characterized by, straightening the
core rod [100] during said collapsing step.
15 2. The method as claimed in claim 1, wherein the straightening of the core
rod [100] comprising the steps of:
a. monitoring hotzone portion [104b/105b] of the substrate tube
[101] in close proximity of the hotzone portion [104b/105b] of the
tube using a monitoring unit [104];
20 b. determining the bend (offset ∆) of the substrate tube [101] from a
set of mid points of said substrate tube [101]; and
c. changing the rotation speed () of the substrate tube [101] based
on bend (offset ∆) of said substrate tube [101].
25 3. The method as claimed in claim 2, wherein said monitoring unit is a
Charged Coupled Device (CCD) Camera [104] with inbuilt software to determine the position of mid point of said substrate tube in the hot zone.
30 4. The method as claimed in claim 2, wherein said monitoring unit [104]
is placed in close proximity of the hotzone [104b/ 105b] of the substrate tube.

5 5. The method as claimed in claim 2, wherein determining the bend (offset
A) of the substrate tube comprises the steps of:
a. determining the edges [109/110] of the substrate tube [101] from
the monitoring unit [104];
b. identifying the mid point of substrate tube by averaging both end
10 of edges [109/110] of the substrate tube [101];
c. obtaining mid point of substrate tube [101] continuously at any
given time; and
d. comparing the average value of previously recorded set of mid
point to the present value of mid point of substrate tube.
15
6. The method as claimed in claim 2, wherein the rotation speed () of the
substrate tube [101] is changed using the following equation:
 = S + g . ∆

wherein,  = actual rotation speed set by the PLC [105] during 20 collapsing step;
S - set rotation speed in the recipe;
g = gain ( a constant); and
∆ = offset (bend).
25 7. A method for preparing straight core rod [100] for manufacturing
optical fiber comprising the steps of:
a. depositing clad and core material inside the substrate tube [101];
b. collapsing the deposited substrate tube to form core rod [100];
and
30 c. straightening the core rod [100] during the collapsing step,
wherein, said straightening step c) characterized by the following steps:

5 i. monitoring hotzone portion [104b/105b] of the substrate tube in
close proximity of the hotzone portion [104b/105b] of the tube
using monitoring unit [104];
ii. determining the bend (offset ∆) of the substrate tube [101] from
the set of mid points of said substrate tube [101]; and
10 iii. changing the rotation speed () of the substrate tube [101] based
on bend (offset ∆) of said substrate tube [101].
8. The method as claimed in claim 7, wherein said monitoring unit is the
Charged Coupled Device (CCD) Camera [104] with inbuilt software to
15 determine the position of mid point of said substrate tube in the hot
zone.
9. The method as claimed in claim 7, wherein said monitoring unit [104]
is placed close proximity of the hotzone [104b/105b] of the substrate
20 tube.
10. The method as claimed in claim 7, wherein determining the bend
(offset ∆) of the substrate tube comprises the steps of:
a. determining the edges [109/110] of the substrate tube [101] from
25 the monitoring unit [104];
b. identifying the mid point of substrate tube by averaging both end
of edges [109/110] of the substrate tube [101];
c. obtaining mid point of substrate tube [101] continuously at any
given time; and
30 d. comparing the average value of previously recorded set of mid
point to the present value of mid point of substrate tube.

11. The method as claimed in claim 7, wherein the rotation speed ()
of the substrate tube [101] is changed using the following equation:
 = S + g. ∆
wherein,  = actual rotation speed set by the PLC [105] during
collapsing step;
S = set rotation speed in the recipe;
g = gain ( a constant); and
∆ = offset (bend).
12. An apparatus for producing straight core rod [100], wherein said
apparatus comprises of:
a. monitoring unit [104] positioned in close proximity of the hotzone
[104b/105b] of the substrate tube [101]; and
b. controlling unit [105] to control the rotation speed of the
substrate tube [101] based on its angular position (offset A);
13. An apparatus as claimed in claim 12, wherein said monitoring unit is charged couple device (CCD) camera [104] with inbuilt software to determine the position of mid point of said substrate tube in the hot zone.
14. An apparatus as claimed in claim 12, wherein said monitoring unit [104] is provided to monitor the hotzone of the substrate tube, identify the edges [109/110] and mid point of substrate tube and to calculate the bend (offset A) of the substrate tube by comparing the average value of previously collected set of mid point to the present value of mid point of the substrate tube.
15. An apparatus as claimed in claim 12, wherein said controlling unit is PLC controller [105] to control the rotation speed of the

substrate tube [101] based on the offset A obtained by the calculating unit.
16. A core rod [100] produced in accordance with any one of the preceding claims.
17. An optical fiber preform produced in accordance with any of claims 1-15.
18. An optical fiber produced in accordance with any of claims 1-15.

METHOD AND APPARATUS FOR MAKING STRAIGHT CORE ROD FOR MANUFACTURING OPTICAL FIBER

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