Title of Invention	METHOD TO CONTROL OUTSIDE DIAMETER OF DEPOSITION TUBE MADE BY CHEMICAL VAPOR DEPOSITION PROCESS
Abstract	The present invention relates to the field of fabricating silica-based optical fiber and more particularly a method and apparatus to control the outside diameter of deposition tube to achieve consistent core refractive index profile using chemical vapor deposition process. The apparatus provided to monitor and control the deposition tube diameter based on the tube temperature, internal tube pressure and glass tube density.

Title of Invention

METHOD TO CONTROL OUTSIDE DIAMETER OF DEPOSITION TUBE MADE BY CHEMICAL VAPOR DEPOSITION PROCESS

Abstract

The present invention relates to the field of fabricating silica-based optical fiber and more particularly a method and apparatus to control the outside diameter of deposition tube to achieve consistent core refractive index profile using chemical vapor deposition process. The apparatus provided to monitor and control the deposition tube diameter based on the tube temperature, internal tube pressure and glass tube density.

Full Text	FORM 2 THE PATENTS ACT 1970 [39 OF 1970] & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See Section 10; rule 13] "METHOD AND APPARATUS TO CONTROL OUTSIDE DIAMETER OF DEPOSITION TUBE MADE BY CHEMICAL VAPOR DEPOSITION PROCESS" STERLITE OPTICAL TECHNOLOGIES LIMITED, Aurangabad, Maharastra, India, of E-2, MIDC, The following specification particularly describes the invention and the manner in which it is to be performed: Sterlite Optical Technologies Limited Confidential copy TITLE OF INVENTION Method and Apparatus to control outside diameter of deposition tube made by chemical vapor deposition process Inventors: Nageswaran Senthil Kumar, Vishal Bhargav Assignee: Sterlite Optical Technologies Limited Correspondence address: E-2, MIDC, Aurangabad, Maharastra, India References Cited: US patent No: 6,502,427 07/01/2003 Michael Z.Yuan, US patent No: 6,105,396 22/08/2000 Glodis et al., 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 Abstract: The present invention relates to the field of fabricating silica-based optical fiber and more particularly a method and apparatus to control the outside diameter of deposition tube to achieve consistent core refractive index profile using chemical vapor deposition process. The apparatus provided to monitor and control the deposition tube diameter based on the tube temperature, internal tube pressure and glass tube density. Sterlite Optical Technologies Limited Confidential copy BACKGROUND OF INVENTION: Field of invention The present invention generally relates to the field of manufacturing optical fiber and more particularly in optical fiber perform having substantially consistent refractive index profile by controlling outside tube diameter of deposition tube. The present invention is to provide a method and apparatus for controlling outside tube diameter of deposition tube. Description of prior art: Conventional method to produce optical fiber performs are carried out by different methods like modified chemical vapor deposition (MCVD), and outer vapor deposition (OVD). Preparation of single mode fiber, multimode fiber and specialty fiber following modified chemical vapor deposition (MCVD) method involves the initial step of preparation of primary perform which involves the chemical gas mixture being injected into the rotating deposition glass tube in the same direction as it is being heated by the traversing heat source. The deposition takes place layer-by-layer glass soot and sintered simultaneously. The deposited glass mainly comprises of clad and core of an optical fiber. The core has slightly high refractive index than clad by means of dopant generally germanium. After the deposition of desired thickness of the glassy materials, the tube is made to collapse by heating the deposition glass tube with a traversing heat source along its length to get solid glass rod called core rod. 2 Page of 18 Sterlite Optical Technologies Limited Confidential copy In conventional method of making preform made by MCVD technique, the tube shrinkage occurs in deposition layers and more in core deposition layer due to external pressure from heat source, which is unable to keep the outside diameter same for each core rod to core rod. Hence the wall thickness of the tube will also change at the end of deposition. The shrinkage rate is not always constant from core rod to core rod due to variation in environment and machine. Hence the end diameter and wall thickness of the tube will not be same for core rod to core rod. The refractive index profile of core is dependent on the gas deposition temperature and the gas flow rate of dopant. The gas deposition temperature is driven by the glass tube temperature and wall thickness of the tube. Even employed with tube temperature control, the refractive index profile of core is dependent on the wall thickness of the tube and tube diameter. Hence, controlling the tube diameter is playing an important role for controlling the core refractive index profile of optical fiber preform. The optical fiber made from specific method like MCVD to get the optical parameters of the fiber like Mode Field Diameter and Cutoff wavelength should meet the customer specification. In order to achieve the specification of optical fiber, the preform made from MCVD should also have the repeatable refractive index of core from each core rod to core rod else otherwise the core rod will be scrapped which will increase the manufacturing cost. To get consistent refractive index profile of core, the outside diameter/wall thickness of the deposition tube during deposition needs to be controlled. 3 Page of 18 Sterlite Optical Technologies Limited Confidential copy Different approaches are undertaken by the manufacturers to meet the above objective specifically to control the outside tube diameter. A simple approach is to keep the inside tube pressure slightly positive than the atmospheric pressure to avoid shrinkage of the tube. In another approach, the deposition tube diameter is monitored and controlled by adjusting inside pressure of the tube. Later method for controlling the outside tube diameter is described in U.S. Pat.No.6105396 and 6502427. The outside diameter controlled by means of varying gas pressure within the deposition tube is disclosed in 6105396. It has been controlled by single variable, which will not provide required accuracy. US Pat No.6502427 describes a dynamic control by using two variables like the flow inside tube as per I) set diameter & measured diameter and II) tube temperature. However this method a drawback while controlling the tube diameter having core layers with higher refractive index as the glass density will change with high dopant concentration. The present invention more specifically attempted to control the outside tube diameter of deposition tube during deposition of both single mode and multimode preform by considering glass density that has not been addressed in prior art. The clad and core deposition of single mode/multimode has different refractive index profile and it is controlled by the concentration of dopant. The concentration of dopant is dependent on the flow rates of reactant gas and temperature of gas. Thus the density of the glass depends on the concentration of dopant. The nominal refractive index of core in multimode (~1.4864) is higher than single mode (-1.46215). The core diameter in single mode is in the range of 8 to 9 jam and where as in multimode the core diameter is in the range of 50 to 62.5 utm and hence the number of deposition layer in multimode 4 Page of 18 Sterlite Optical Technologies Limited Confidential copy preform is higher than the single mode preform. Thus more care needs to be taken while making multimode preform. The clad and core dopant are entirely different and hence the density of glass, the pressure inside the tube to control will not be same for clad and core for controlling the same diameter. The outside tube diameter is controlled by means of excess pressure than the atmospheric pressure. Basically, glass expansion or shrinkage of the tube depends on thermal expansion coefficient of glass, which depends on glass density, glass temperature and pressure difference in and outside of the glass. The present invention dynamically controls the outside diameter of deposition tube by considering three parameters viz. glass density, glass temperature and pressure inside the tube. The density of the glass varies according to the composition of glass. Thus it is related with (dopant) refractive index of glass which is familiar to persons well known to this present art. Following equation (1) shows the thermal expansion coefficient of glass. p = - (l/p)(dp/dT)(P) (1) where p is density of glass T is temperature of glass and P is pressure inside the glass. d p is change in density by change in temperature d T From the above equation, it is well known that the glass expansion is depends on glass density, temperature and pressure. The present invention generally relates to method and apparatus to control the desired outside tube diameter of the deposition tube, which further keeps in control the wall thickness of the tube during deposition, thus the refractive index of core rod is controlled for continuous production 5 Page of 18 Sterlite Optical Technologies Limited Confidential copy Objects of the invention The object of the present invention is to provide a method and apparatus for controlling the outside tube diameter of deposition tube during vapor deposition process by considering the three parameters viz. glass density, glass temperature and pressure inside the tube. Another objective of the present invention is to get the repeatable core refractive index profile of the core rods made by method like MCVD technique. Yet another objective of the present invention is to have the dynamic control of outside tube diameter of deposition tube of all types of preform manufactured in MCVD process like single mode and multimode process. Still another objective of the present invention is to have accurate control in clad and core deposition of deposition process by considering the concentration of dopant and thus density of glass. 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: Fig. 1 is a schematic representation of the glass-working lathe on which the deposition of the deposition tube takes place with apparatus of the present invention attached. Fig. 2 is a flow diagram of control of diameter of the outside tube diameter Fig. 3 is an example curve represents the A Pressure change Vs Diameter error. 6 Page of 18 Sterlite Optical Technologies Limited Confidential copy Fig. 4 is a schematic representation of the field of view of the CCD camera presented on a cartesian plane. Fig. 5 is a plot of the intensity variation as a function of radial position of deposition tube. Fig. 6 is a plot of the change in light intensity along the radial position of deposition tube. DETAILED DESCRIPTION OF THE PRESENT INVENTION The deposition process of a deposition tube is schematically presented in figure 1. The deposition tube 101 is held between two chucks of a glass-working lathe (not shown) and rotated about its longitudinal axis. A flame torch 103, being mounted on a movable bench 102, is placed underneath the deposition tube 101 so as to heat the latter. Also diameter-monitoring device preferably charge coupled device (CCD) camera 104 and temperature monitoring device 105 preferably Infrared pyrometer are mounted on the movable bench 102. The mounting of the torch 103, camera 104 and temperature-monitoring device 105 ensures the movement of all these entities to traverse at with same speed. The apparatus of the present invention consists of a monitoring units and controlling units of diameter, temperature and pressure of the tube. The pressure-measuring device 106 is placed on the soot-collecting chamber 108, which is connected with the deposition tube 101 to measure the pressure inside the tube. The Camera 104 and temperature monitoring 105 devices is placed 110 near the hot zone of the tube, to measure the diameter and temperature over the hot zone position. 7 Page of 18 Sterlite Optical Technologies Limited Confidential copy The reactant gases are injected at 111 in a controlled way inside the deposition tube 101 and reaction occurs near the hot zone 110 to form as a soot material to deposit inside the deposition tube 101. There is an exhaust flexible tube (109) in soot box chamber is connected with soot scrubbing system. The non-deposited soot and gases are evacuated thru the exhaust conduit (109) to the soot scrubbing system. The deposition takes places along the direction of movement of hot zone from the Left (L) to Right (R). After the desired layer of thickness deposition, the deposited tube is heated with higher temperature with lower movable bench speed to enable to collapse the deposited tube to form as a core rod. The core rod is then overcladded to form glass preform, which is suitable to form an optical fiber with desired parameters. There is a table called recipe to modify parameters to get the desired refractive index profile and desired glass material deposition. In this table, required diameter, required temperature of the tube and required reactant flows in each layer of deposition are fed and the same data is transferred to the Programmable Logic control (PLC) to control the diameter as mentioned in the Flow diagram 2. All the data is fed and calculated in the order of milliseconds. The diameter control receives the predetermined set diameter (Dl) tube from the table as said and measured diameter (D2) from the Camera (104). The diameter error (AD) = D1-D2 is calculated to measure the (AP) pressure to the pressure controller which controls the pressure inside the tube. The value of AD can be both positive and negative and accordingly AP will also have the same sign. The (AP) Del pressure is calculated as following equation (2) and example is provided in Fig 3. 8 Page of 18 Sterlite Optical Technologies Limited Confidential copy AP = SIGN of (ΔD) x Kl x (ΔD)^K2 (2) Kl, K2 are constants that represent gain and exponential. Kl, K2 constants are selected based on the start tube diameter and wall thickness of the tube. The flow rate of reactant gas are fed from the table (recipe) to PLC which controls the dopant concentration and the said PLC has a separate algorithm to calculate the Start pressure (Ps) for each layer of deposition. The Start pressure (Ps) of each layer is calculated by considering three parameters like flow rate of reactant gases, temperature of the tube and to the diameter error (AD) of last layer. The Start pressure (Ps) is proportional to the temperature of the tube (T), diameter error (AD) of last layer and inversely proportional to the reactant gas flow rates, which control the dopant concentration. The Start pressure (Ps), A P pressure and measured pressure inside the soot box chamber are fed to the pressure control which controls the total pressure required inside the soot box chamber by means of adjusting the flow rate of inert gas preferably nitrogen goes in to soot box chamber (107). The soot box chamber is connected with the deposition tube (101). The pressure control has three constant like Kp, KD and Ki and the constants are changed to get required performance of the equipment. By summing up the Start pressure (Ps) and A Pressure, the total pressure required inside the tube is calculated as shown equation (3). Total Pressure = Ps + A P (3) Hence the total pressure required inside the tube is calculated instantaneously according to the measured diameter, measured temperature and measured flow rates of reactant gases. 9 Page of 18 Sterlite Optical Technologies Limited Confidential copy The working of the diameter-monitoring unit can be described using figure 4-6. The figure 4 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 deposition tube 104 in the hot zone as seen against a dark background 119. 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 115 is selected. The intensity of the light on each photosites of the array 115 is analyzed. A plot of light intensity as sensed by the photosites in the selected array 115 is plotted against their position along the y-direction as shown in figure 5. Figure 5 is characterized by a low intensity region 118 and 119 and high intensity region 120, with regions 120 and 114 showing a sudden change in the light intensity. The low intensity region 118 and 119 represent dark background whereas the high intensity region 120 represent the deposition tube 104 in the hot zone. The region of sudden change in the intensity of radiation 120 and 114 represents the bottom and top edge of the deposition tube 104 respectively. In order to assign the coordinate value to the deposition tube edges a plot of change in radiation intensity with change in radial position along the y-direction is plotted against the radial position of the deposition tube 104 (figure 6). In this figure, two points of highest gradient intensity 120 and 114 are observed. The radial position corresponding to the highest gradient intensity 116 and 117 represents the edge of the deposition tube. The difference between the two edges 116 and 117 is the diameter of the deposition tube. 10 Page of 18 Sterlite Optical Technologies Limited Confidential copy The diameter measurement accuracy is also very important to have better control in tube diameter of the deposition tube. The above Camera has inbuilt software which has considered the intensity of heat of the tube to get the accuracy to measure in the order 0.01 mm of the diameter. While measuring at hot zone of the tube, the intensity of the hot zone has been considered to give better accuracy of measurement. According to the present invention, the above said method has considered the diameter, temperature, density of tube and pressure inside tube which provides the required accuracy in tube diameter control and hence the wall thickness of the tube is maintained from core rod to core rod which is the driving factor for controlling the refractive index profile of the core rod. While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only and should not limit the scope of the invention set forth in the following claims. 11 Page of 18 Sterlite Optical Technologies Limited Confidential copy We Claim: 1. A method and apparatus for controlling the outside tube diameter of deposition tube during deposition process and the said method comprising the steps of: (a) determining the total pressure required inside the tube to obtain the desired outside tube diameter, wherein said total pressure is determined based on dopant concentration of glass tube, measured tube diameter, predetermined set diameter and measured tube temperature; (b) determining the flow rate of inert gas required inside the said tube is based on the measured pressure and total pressure (P) required inside the tube; (c) injecting said inert gas inside the soot box chamber, which is connected with said deposition tube thereby controlling the required total pressure inside the tube. 2. The method in claim 1 further comprising of: a. measuring the tube diameter near the hot zone of the tube by using camera with high accuracy preferably b. calculating the diameter error by calculating the difference between the set and measured diameter; c. calculating the change in A Pressure (AP) inside the tube by using the diameter error; 3. The method in claim 1 further comprises of: calculating start pressure (Ps) inside the tube for every deposition layers by considering the dopant concentration of glass tube, measured tube temperature and diameter error of last layers. 12 Page of 18 Sterlite Optical Technologies Limited Confidential copy 4. The method in claim 3, wherein said Start pressure (Ps) is added with said A Pressure (A P) to get the total pressure (P) required inside the tube. 5. The method in claim 3, wherein said tube temperature is measured at near the hot zone of tube by using temperature-measuring device. 6. The method in claim 3, wherein said dopant concentration of glass tube is determined by considering the dopant gases flow rate inside the tube. 7. The method in claim 4, wherein said Total pressure (P) is controlled by adjusting the flow rate of gas in to the soot box chamber and considering the measured pressure inside the soot box chamber. 8. The method in claim 7, wherein the said measured pressure is measured by pressure measuring unit device, which is connected with soot box chamber. 9. The method of controlling the outside tube diameter of the deposition tube comprising the steps of; a) determining the A Pressure by considering diameter error; b) calculating said A Pressure by considering two constants Kl and K2, which is determined by the initial deposition tube diameter and wall thickness of deposition tube; . c)determining the start pressure (Ps) by considering (a) measured tube temperature from temperature measuring device (b) diameter error of last layer (c) dopant gases flow rate. 13 Page of 18 Sterlite Optical Technologies Limited Confidential copy 10. The method in claim 9, wherein said outside tube diameter of deposition tube is dynamically controlled thereby controlling the wall thickness of the tube to achieve the required core refractive index profile from core rod to core rod. 11. A method and apparatus for controlling the outside tube diameter of deposition tube during deposition process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings. Dated this 23rd day of February, 2005 14 Page of 18

Full Text

FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See Section 10; rule 13]
"METHOD AND APPARATUS TO CONTROL OUTSIDE DIAMETER OF DEPOSITION TUBE MADE BY CHEMICAL VAPOR DEPOSITION PROCESS"

STERLITE OPTICAL TECHNOLOGIES LIMITED, Aurangabad, Maharastra, India,

of E-2, MIDC,

The following specification particularly describes the invention and the manner in which it is to be performed:

Sterlite Optical Technologies Limited

Confidential copy

TITLE OF INVENTION
Method and Apparatus to control outside diameter of deposition tube
made by chemical vapor deposition process
Inventors: Nageswaran Senthil Kumar, Vishal Bhargav
Assignee: Sterlite Optical Technologies Limited
Correspondence address: E-2, MIDC, Aurangabad, Maharastra, India
References Cited:
US patent No: 6,502,427 07/01/2003 Michael Z.Yuan,
US patent No: 6,105,396 22/08/2000 Glodis et al.,
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
Abstract:
The present invention relates to the field of fabricating silica-based optical fiber and more particularly a method and apparatus to control the outside diameter of deposition tube to achieve consistent core refractive index profile using chemical vapor deposition process. The apparatus provided to monitor and control the deposition tube diameter based on the tube temperature, internal tube pressure and glass tube density.

Sterlite Optical Technologies Limited

Confidential copy

BACKGROUND OF INVENTION:
Field of invention
The present invention generally relates to the field of manufacturing optical fiber and more particularly in optical fiber perform having substantially consistent refractive index profile by controlling outside tube diameter of deposition tube. The present invention is to provide a method and apparatus for controlling outside tube diameter of deposition tube.
Description of prior art:
Conventional method to produce optical fiber performs are carried out by different methods like modified chemical vapor deposition (MCVD), and outer vapor deposition (OVD). Preparation of single mode fiber, multimode fiber and specialty fiber following modified chemical vapor deposition (MCVD) method involves the initial step of preparation of primary perform which involves the chemical gas mixture being injected into the rotating deposition glass tube in the same direction as it is being heated by the traversing heat source. The deposition takes place layer-by-layer glass soot and sintered simultaneously. The deposited glass mainly comprises of clad and core of an optical fiber. The core has slightly high refractive index than clad by means of dopant generally germanium. After the deposition of desired thickness of the glassy materials, the tube is made to collapse by heating the deposition glass tube with a traversing heat source along its length to get solid glass rod called core rod.
2
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

In conventional method of making preform made by MCVD technique, the tube shrinkage occurs in deposition layers and more in core deposition layer due to external pressure from heat source, which is unable to keep the outside diameter same for each core rod to core rod. Hence the wall thickness of the tube will also change at the end of deposition. The shrinkage rate is not always constant from core rod to core rod due to variation in environment and machine. Hence the end diameter and wall thickness of the tube will not be same for core rod to core rod. The refractive index profile of core is dependent on the gas deposition temperature and the gas flow rate of dopant. The gas deposition temperature is driven by the glass tube temperature and wall thickness of the tube. Even employed with tube temperature control, the refractive index profile of core is dependent on the wall thickness of the tube and tube diameter. Hence, controlling the tube diameter is playing an important role for controlling the core refractive index profile of optical fiber preform.
The optical fiber made from specific method like MCVD to get the optical parameters of the fiber like Mode Field Diameter and Cutoff wavelength should meet the customer specification. In order to achieve the specification of optical fiber, the preform made from MCVD should also have the repeatable refractive index of core from each core rod to core rod else otherwise the core rod will be scrapped which will increase the manufacturing cost. To get consistent refractive index profile of core, the outside diameter/wall thickness of the deposition tube during deposition needs to be controlled.
3
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

Different approaches are undertaken by the manufacturers to meet the above objective specifically to control the outside tube diameter. A simple approach is to keep the inside tube pressure slightly positive than the atmospheric pressure to avoid shrinkage of the tube. In another approach, the deposition tube diameter is monitored and controlled by adjusting inside pressure of the tube. Later method for controlling the outside tube diameter is described in U.S. Pat.No.6105396 and 6502427.
The outside diameter controlled by means of varying gas pressure within the deposition tube is disclosed in 6105396. It has been controlled by single variable, which will not provide required accuracy.
US Pat No.6502427 describes a dynamic control by using two variables like the flow inside tube as per I) set diameter & measured diameter and II) tube temperature. However this method a drawback while controlling the tube diameter having core layers with higher refractive index as the glass density will change with high dopant concentration.
The present invention more specifically attempted to control the outside tube diameter of deposition tube during deposition of both single mode and multimode preform by considering glass density that has not been addressed in prior art. The clad and core deposition of single mode/multimode has different refractive index profile and it is controlled by the concentration of dopant. The concentration of dopant is dependent on the flow rates of reactant gas and temperature of gas. Thus the density of the glass depends on the concentration of dopant. The nominal refractive index of core in multimode (~1.4864) is higher than single mode (-1.46215). The core diameter in single mode is in the range of 8 to 9 jam and where as in multimode the core diameter is in the range of 50 to 62.5 utm and hence the number of deposition layer in multimode
4
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

preform is higher than the single mode preform. Thus more care needs to be taken while making multimode preform. The clad and core dopant are entirely different and hence the density of glass, the pressure inside the tube to control will not be same for clad and core for controlling the same diameter.
The outside tube diameter is controlled by means of excess pressure than the atmospheric pressure. Basically, glass expansion or shrinkage of the tube depends on thermal expansion coefficient of glass, which depends on glass density, glass temperature and pressure difference in and outside of the glass. The present invention dynamically controls the outside diameter of deposition tube by considering three parameters viz. glass density, glass temperature and pressure inside the tube. The density of the glass varies according to the composition of glass. Thus it is related with (dopant) refractive index of glass which is familiar to persons well known to this present art. Following equation (1) shows the thermal expansion coefficient of glass.
p = - (l/p)(dp/dT)(P) (1)
where p is density of glass
T is temperature of glass
and P is pressure inside the glass.
d p is change in density by change in temperature d T
From the above equation, it is well known that the glass expansion is depends on glass density, temperature and pressure.
The present invention generally relates to method and apparatus to control the desired outside tube diameter of the deposition tube, which further keeps in control the wall thickness of the tube during deposition, thus the refractive index of core rod is controlled for continuous production
5
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

Objects of the invention
The object of the present invention is to provide a method and apparatus for controlling the outside tube diameter of deposition tube during vapor deposition process by considering the three parameters viz. glass density, glass temperature and pressure inside the tube.
Another objective of the present invention is to get the repeatable core refractive index profile of the core rods made by method like MCVD technique.
Yet another objective of the present invention is to have the dynamic control of outside tube diameter of deposition tube of all types of preform manufactured in MCVD process like single mode and multimode process. Still another objective of the present invention is to have accurate control in clad and core deposition of deposition process by considering the concentration of dopant and thus density of glass.
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:
Fig. 1 is a schematic representation of the glass-working lathe on which
the deposition of the deposition tube takes place with apparatus of the
present invention attached.
Fig. 2 is a flow diagram of control of diameter of the outside tube
diameter
Fig. 3 is an example curve represents the A Pressure change Vs
Diameter error.
6
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

Fig. 4 is a schematic representation of the field of view of the CCD
camera presented on a cartesian plane.
Fig. 5 is a plot of the intensity variation as a function of radial position
of deposition tube.
Fig. 6 is a plot of the change in light intensity along the radial position
of deposition tube.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The deposition process of a deposition tube is schematically presented in figure 1. The deposition tube 101 is held between two chucks of a glass-working lathe (not shown) and rotated about its longitudinal axis. A flame torch 103, being mounted on a movable bench 102, is placed underneath the deposition tube 101 so as to heat the latter. Also diameter-monitoring device preferably charge coupled device (CCD) camera 104 and temperature monitoring device 105 preferably Infrared pyrometer are mounted on the movable bench 102. The mounting of the torch 103, camera 104 and temperature-monitoring device 105 ensures the movement of all these entities to traverse at with same speed.
The apparatus of the present invention consists of a monitoring units and controlling units of diameter, temperature and pressure of the tube. The pressure-measuring device 106 is placed on the soot-collecting chamber 108, which is connected with the deposition tube 101 to measure the pressure inside the tube. The Camera 104 and temperature monitoring 105 devices is placed 110 near the hot zone of the tube, to measure the diameter and temperature over the hot zone position.
7
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

The reactant gases are injected at 111 in a controlled way inside the deposition tube 101 and reaction occurs near the hot zone 110 to form as a soot material to deposit inside the deposition tube 101. There is an exhaust flexible tube (109) in soot box chamber is connected with soot scrubbing system. The non-deposited soot and gases are evacuated thru the exhaust conduit (109) to the soot scrubbing system. The deposition takes places along the direction of movement of hot zone from the Left (L) to Right (R). After the desired layer of thickness deposition, the deposited tube is heated with higher temperature with lower movable bench speed to enable to collapse the deposited tube to form as a core rod. The core rod is then overcladded to form glass preform, which is suitable to form an optical fiber with desired parameters.
There is a table called recipe to modify parameters to get the desired refractive index profile and desired glass material deposition. In this table, required diameter, required temperature of the tube and required reactant flows in each layer of deposition are fed and the same data is transferred to the Programmable Logic control (PLC) to control the diameter as mentioned in the Flow diagram 2. All the data is fed and calculated in the order of milliseconds.
The diameter control receives the predetermined set diameter (Dl) tube from the table as said and measured diameter (D2) from the Camera (104). The diameter error (AD) = D1-D2 is calculated to measure the (AP) pressure to the pressure controller which controls the pressure inside the tube. The value of AD can be both positive and negative and accordingly AP will also have the same sign. The (AP) Del pressure is calculated as following equation (2) and example is provided in Fig 3.
8
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

AP = SIGN of (ΔD) x Kl x (ΔD)^K2 (2)
Kl, K2 are constants that represent gain and exponential. Kl, K2 constants are selected based on the start tube diameter and wall thickness of the tube. The flow rate of reactant gas are fed from the table (recipe) to PLC which controls the dopant concentration and the said PLC has a separate algorithm to calculate the Start pressure (Ps) for each layer of deposition. The Start pressure (Ps) of each layer is calculated by considering three parameters like flow rate of reactant gases, temperature of the tube and to the diameter error (AD) of last layer. The Start pressure (Ps) is proportional to the temperature of the tube (T), diameter error (AD) of last layer and inversely proportional to the reactant gas flow rates, which control the dopant concentration.
The Start pressure (Ps), A P pressure and measured pressure inside the soot box chamber are fed to the pressure control which controls the total pressure required inside the soot box chamber by means of adjusting the flow rate of inert gas preferably nitrogen goes in to soot box chamber (107). The soot box chamber is connected with the deposition tube (101). The pressure control has three constant like Kp, KD and Ki and the constants are changed to get required performance of the equipment. By summing up the Start pressure (Ps) and A Pressure, the total pressure required inside the tube is calculated as shown equation (3).
Total Pressure = Ps + A P (3)
Hence the total pressure required inside the tube is calculated instantaneously according to the measured diameter, measured temperature and measured flow rates of reactant gases.
9
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

The working of the diameter-monitoring unit can be described using figure 4-6. The figure 4 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 deposition tube 104 in the hot zone as seen against a dark background 119. 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 115 is selected. The intensity of the light on each photosites of the array 115 is analyzed.
A plot of light intensity as sensed by the photosites in the selected array 115 is plotted against their position along the y-direction as shown in figure 5. Figure 5 is characterized by a low intensity region 118 and 119 and high intensity region 120, with regions 120 and 114 showing a sudden change in the light intensity. The low intensity region 118 and 119 represent dark background whereas the high intensity region 120 represent the deposition tube 104 in the hot zone. The region of sudden change in the intensity of radiation 120 and 114 represents the bottom and top edge of the deposition tube 104 respectively. In order to assign the coordinate value to the deposition tube edges a plot of change in radiation intensity with change in radial position along the y-direction is plotted against the radial position of the deposition tube 104 (figure 6).
In this figure, two points of highest gradient intensity 120 and 114 are observed. The radial position corresponding to the highest gradient intensity 116 and 117 represents the edge of the deposition tube. The difference between the two edges 116 and 117 is the diameter of the deposition tube.
10
Page of 18

Sterlite Optical Technologies Limited

Confidential copy

The diameter measurement accuracy is also very important to have better control in tube diameter of the deposition tube. The above Camera has inbuilt software which has considered the intensity of heat of the tube to get the accuracy to measure in the order 0.01 mm of the diameter. While measuring at hot zone of the tube, the intensity of the hot zone has been considered to give better accuracy of measurement.
According to the present invention, the above said method has considered the diameter, temperature, density of tube and pressure inside tube which provides the required accuracy in tube diameter control and hence the wall thickness of the tube is maintained from core rod to core rod which is the driving factor for controlling the refractive index profile of the core rod.
While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only and should not limit the scope of the invention set forth in the following claims.
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We Claim:
1. A method and apparatus for controlling the outside tube diameter
of deposition tube during deposition process and the said method
comprising the steps of:
(a) determining the total pressure required inside the tube to obtain the desired outside tube diameter, wherein said total pressure is determined based on dopant concentration of glass tube, measured tube diameter, predetermined set diameter and measured tube temperature;
(b) determining the flow rate of inert gas required inside the said tube is based on the measured pressure and total pressure (P) required inside the tube;
(c) injecting said inert gas inside the soot box chamber, which is connected with said deposition tube thereby controlling the required total pressure inside the tube.
2. The method in claim 1 further comprising of:
a. measuring the tube diameter near the hot zone of the tube
by using camera with high accuracy preferably b. calculating the diameter error by calculating the difference
between the set and measured diameter;
c. calculating the change in A Pressure (AP) inside the tube by
using the diameter error;
3. The method in claim 1 further comprises of:
calculating start pressure (Ps) inside the tube for every deposition layers by considering the dopant concentration of glass tube, measured tube temperature and diameter error of last layers.
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4. The method in claim 3, wherein said Start pressure (Ps) is added with said A Pressure (A P) to get the total pressure (P) required inside the tube.
5. The method in claim 3, wherein said tube temperature is measured at near the hot zone of tube by using temperature-measuring device.
6. The method in claim 3, wherein said dopant concentration of glass tube is determined by considering the dopant gases flow rate inside the tube.
7. The method in claim 4, wherein said Total pressure (P) is controlled by adjusting the flow rate of gas in to the soot box chamber and considering the measured pressure inside the soot box chamber.
8. The method in claim 7, wherein the said measured pressure is
measured by pressure measuring unit device, which is connected
with soot box chamber.
9. The method of controlling the outside tube diameter of the
deposition tube comprising the steps of;
a) determining the A Pressure by considering diameter error;
b) calculating said A Pressure by considering two constants Kl and K2, which is determined by the initial deposition tube diameter and wall thickness of deposition tube; .
c)determining the start pressure (Ps) by considering (a) measured tube temperature from temperature measuring device (b) diameter error of last layer (c) dopant gases flow rate.
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10. The method in claim 9, wherein said outside tube diameter of deposition tube is dynamically controlled thereby controlling the wall thickness of the tube to achieve the required core refractive index profile from core rod to core rod.
11. A method and apparatus for controlling the outside tube diameter of deposition tube during deposition process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
Dated this 23rd day of February, 2005

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METHOD TO CONTROL OUTSIDE DIAMETER OF DEPOSITION TUBE MADE BY CHEMICAL VAPOR DEPOSITION PROCESS

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