Title of the Invention:

Apparatus & Method for drawing Optical Fiber having desired Waveguide parameters and Fiber produced thereby. Field of the Invention:

The present invention relates to an apparatus and method for drawing an optical fiber having desired waveguide parameters and fiber produced thereby. Particularly, the present invention relates to an apparatus and method for avoiding change in fiber tension during drawing of an optical fiber so that the fiber drawn and produced has desired waveguide parameters. The present invention also relates to an optical fiber having desired waveguide parameters. Background of the Invention:

Optical fibers are inherently versatile as a transmission medium for all forms of information, be it voice, video or data. Optical fiber comprises a core, to which essentially the entire signal is confined, and a clad surrounding the core. The optical fiber is manufactured in a way to have core with higher refractive index in order to achieve light transmission inside the core region. The optical power also spreads in the cladding region near the core region.

The optical fibers [hereinafter may be referred to as fiber] for telecommunication are required to operate with desired waveguide parameters, for example cut-off wavelength, chromatic dispersion and modified field diameter [MFD]. As the requirement for optical performance of optical fibers is stringent, the desired waveguide parameters, such as cut-off wavelength, chromatic dispersion and MFD in optical fiber needs to be properly controlled and maintained at a desired value or within desired range. However, certain physical and chemical constraints in the process for drawing the fiber from an optical fiber preform [herein after may be referred to as preform] can result in change in desired values of waveguide parameters, that is, desired values of cut-off wavelength, chromatic dispersion and MFD parameters of the fiber. One of the commonly known constraints known to cause change in desired values of waveguide parameters, that is in desired values of cut-off wavelength, chromatic dispersion and MFD parameters of the fiber is refractive index profile of the fiber, particularly refractive index of core of the fiber.

It has been observed that the change in fiber tension during drawing of a fiber is responsible for causing change in stress in the core region of the fiber,

which causes change in refractive index profile of the fiber, particularly refractive index of core of the fiber meaning thereby change in desired values of waveguide parameters, that is change in desired values of cut-off wavelength, chromatic dispersion and MFD parameters of the fiber, and hence the fiber produced cannot be used for desired applications.

It has also been observed that the change in fiber tension is generally caused by variations in preform diameter, feeding speed of preform, draw speed of fiber and temperature of the preform inside the furnace. Therefore, if preform diameter, feeding speed of preform, draw speed of fiber and temperature of the preform inside the furnace are controlled or maintained at particular value or within desired range, then change in fiber tension can be controlled or maintained at particular value or within a particular range meaning thereby change in stress in the core region of the fiber, that is, change in refractive index profile of the fiber, particularly refractive index of core of the fiber, and hence change in desired values of waveguide parameters, that is change in desired values of cut-off wavelength, chromatic dispersion and MFD parameters of the fiber can be controlled and maintained within predetermined range or at predetermined value. Accordingly, the fiber produced will be suitable for use in desired applications.

However, it has been observed that change in temperature of the preform inside the furnace is primarily responsible for causing change in fiber tension during the fiber draw process, which has been found to be responsible for causing change in stress in the core region of the fiber, which in-turn is responsible for causing change in refractive index profile of the fiber, particularly refractive index of core of the fiber meaning thereby causing change in desired values of waveguide parameters, that is change in desired values of cut-off wavelength, chromatic dispersion and MFD parameters of the fiber, and hence, rendering the fiber produced un-suitable for desired applications. Accordingly, a need arises for having a method and apparatus for suitably controlling and maintaining temperature of the preform inside the furnace.

The prior art teaches a method and apparatus for suitably controlling and maintaining temperature of the preform inside the furnace for avoiding a change in fiber tension during the process of drawing a fiber, wherein a

tensionometer is provided for constantly and repeatedly measuring tension of the fiber drawn at a point outside the furnace employed. In accordance with this method the tension measured is fed to a controller which in-turn controls and maintains temperature of the preform inside the furnace which results in control and maintenance of fiber tension at or around a desired value.

It has been observed by the present inventors that surface of the preform which is outside the furnace dissipates heat meaning thereby there is heat loss which has been found to cause change in temperature of preform inside the furnace. Accordingly, merely by constantly and repeatedly measuring fiber tension of the drawn fiber at a point outside the furnace at regular intervals and feeding the measured value of fiber tension to a controller for controlling and maintaining temperature of the preform inside the furnace cannot control change in temperature of the preform inside the furnace which as observed by present inventors is primarily caused due to heat loss from that surface of the preform which is outside the furnace meaning thereby cannot control change in fiber tension caused due to this reason. It has been further observed that such heat loss from preform surface continues till the preform reaches inside the furnace. Accordingly, the known prior art has not been found suitable to substantially avoid change in fiber tension during the fiber draw process.

This problem is better understood when referring to accompanying Figure 1. In accordance with known prior art, the furnace 1 comprising a furnace chamber 2 provided with heating means 3 having heating elements 4 is provided with a preform 5 having a bottom end 6 and a top end 7. The preform 5 is suitably suspended from a suspension means 8 with the help of a handle rod 9 in a manner that its bottom end 6 coincides with center of the heating means 3 so that the preform tip 10 can be suitably heated to a temperature suitable for drawing of a fiber 11.

In accordance with known prior art, fiber tension is constantly and repeatedly measured at regular intervals with the help of a tensionometer 12 provided at a point 13 outside the furnace 1. The measured value of fiber tension is fed to a controller 14 for controlling and maintaining temperature of the preform 5 inside the furnace 1 which results in control and maintenance of fiber tension at or around a desired value only based on input of change of fiber tension of the fiber which has been already drawn.

As described herein above, the present inventors have observed that heat loss continuously takes from surface of the preform 5, as shown by arrows 15 [Figure 1], which has been surprisingly observed to adversely effect temperature of the preform 5 inside the furnace 1, that is has been observed to cause change in temperature of the preform 5 inside the furnace 1. Accordingly, the heat loss from surface of the preform 5 also causes change in fiber tension in the drawn fiber.

It is understood from the foregoing description that the known prior art is only suitable to avoid fiber tension caused due to change in heating temperature of preform inside the furnace, and such change is corrected only based on input of change in fiber tension of the drawn fiber, that is when a damage has actually been caused.

It is also understood from the foregoing description that the known prior art is not at all suitable for controlling and maintaining change in temperature of the preform inside the furnace when it is caused due to heat loss from surface of the perform which is outside of the furnace, meaning thereby the known prior art is not at all suitable for avoiding change in fiber tension which is being caused due to heat loss from surface of the perform which is outside of the furnace.

It is clear from description of the known prior art that it cannot control and maintain change in temperature of the preform inside the furnace when it is caused due to heat loss from surface of the preform which is outside of the furnace meaning thereby it cannot control and maintain change in fiber tension when it is caused due to heat loss from surface of the preform which is outside of the furnace. As such heat loss from preform surface continues till the preform 5 completely reaches inside the furnace 1, at least till the closing means 16 provided at the joining point of top end 7 of the preform 5 and handle rod 9 closes top opening of the furnace 1. Therefore, the control and maintenance of change in temperature of preform inside the furnace caused due to heat loss from surface of the perform which is outside of the furnace becomes essential for substantially avoiding change in fiber tension which otherwise will be caused in the drawn fiber, that is it is essential to control change in fiber tension before it has been actually caused.

The another drawback of the known prior art is that it is not at all suitable to control sudden variation in temperature of the preform which happens when the preform completely enters the furnace meaning thereby it is not at all suitable to avoid sudden deviation and variation in fiber tension which happens when the preform completely enters the furnace. The accompanying Figure 4a illustrates such sudden variation in fiber tension, wherein graph 401 illustrates change in fiber tension with movement of preform inside the furnace and graph 402 illustrates ideal graph of fiber tension with respect to preform length outside the furnace which is ideally desired to be constant during fiber draw process.

The Japanese patent JP 63-123832 laid open, makes an attempt to overcome above-described problems and drawbacks of prior art by providing a spectroscopic means in direct contact with and at the top end of the perform which is outside the furnace, but to measure the wavelength of the energy radiated by the perform in side the furnace. The main drawback of this system is as of above discussed prior art, that is it relies on wavelength of energy radiated from that of the perform which is inside the furnace and as the spectroscopic means is in direct contact with and on the top end of the perform it gets damaged due to heating when preform is about to enter the furnace. Therefore, it has been observed that even the system taught in JP 63-123832 does not substantially overcome above discussed problems and drawbacks of the prior art. Need of the Invention: Therefore, there is need to have an apparatus and method for drawing an optical fiber which substantially avoids change in fiber tension by controlling change in temperature of the preform inside the furnace based on change in temperature of surface of that part of the preform which is outside of the furnace, that is before the fiber has actually been drawn, and also substantially avoids sudden change in fiber tension by controlling sudden variation in temperature of the preform when it completely enters the furnace. There is also a need of the apparatus and method for drawing an optical fiber which is not only suitable to measure the temperature of preform but also does not get damage by heat losses from the preform or the furnace, and hence has long life, and is optionally suitable for coupling with existing furnaces.

Objects of the Invention:

Accordingly, the main object of the present invention is to provide an apparatus and method for drawing an optical fiber which substantially avoids change in fiber tension by controlling change in temperature of the preform inside the furnace based on change in temperature of surface of that part of the preform which is outside the furnace, that is, which is capable of substantially avoiding change in fiber tension before the fiber has actually been drawn, and is suitable for controlling and maintaining the fiber tension during entire drawing process so that the fiber drawn and produced thereby has desired waveguide parameters, particularly the cut-off wavelength.

Another object of the present invention is to provide an apparatus and method for drawing an optical fiber which is suitable for substantially avoiding sudden change in fiber tension by controlling sudden variation in temperature of the preform when it completely enters the furnace.

Still another particular object of the present invention is to provide an apparatus and method for drawing an optical fiber wherein the temperature measurement means is neither in direct contact with the preform nor on the top end thereof, and hence it does not get damaged due to heating when preform is about to enter the furnace or when it has completely entered the furnace, and hence has long life..

Yet another particular object of the present invention is to provide an apparatus and method for drawing an optical fiber which is not only suitable for measuring temperature of preform when it is partly outside the furnace, but is also suitable for coupling with existing furnaces.

This is also an object of the present invention to provide an optical fiber having desired waveguide parameters, particularly cut-off wavelength.

Other objects, advantages and preferred embodiments of the present invention will be apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit scope of the present invention, but are incorporated merely for illustrating the present invention. Brief Description of the Accompanying Figures:

Figure 1 illustrates apparatus and method for drawing an optical fiber by controlling change in temperature of preform inside the furnace based on change in fiber tension in accordance with prior art. Figure 2 illustrates apparatus and method for drawing an optical fiber by controlling change in temperature of preform inside the furnace based on heat loss from surface of the preform in accordance with one of the preferred embodiments of the present invention.

Figure 3 illustrates apparatus and method for drawing an optical fiber by controlling change in temperature of preform inside the furnace when the preform is completely inside the furnace in accordance with another preferred embodiment of the present invention.

Figure 4a illustrates graphic presentation of change [variations] in fiber tension while drawing a fiber wherein graph 401 illustrates change in fiber tension with movement of preform inside the furnace particularly illustrates sudden change in fiber tension when the preform has completely entered inside the furnace, and graph 402 illustrates ideal graph of fiber tension with respect to preform length outside the furnace which is as desired to be constant during fiber draw process, and graph 403 illustrates graph of fiber tension in accordance with one of the preferred embodiment of the present invention.

Figure 4b illustrates graphic presentation of change [variations] in heat loss with movement of preform inside the furnace particularly illustrates sudden drop in heat loss when the preform has completely entered inside the furnace. Detailed Description of the Invention;

It is apparent form the foregoing description that the apparatuses and methods known in the prior art do not satisfactorily substantially avoid change in fiber tension during entire drawing of an optical fiber, and suffer from various problems, disadvantages and drawbacks as described herein. It is also understood from the foregoing description that to satisfy the stringent requirements of performance of optical fibers, the desired waveguide parameters, particularly cut-off wavelength in optical fiber should be suitably controlled and maintained while drawing the fiber so that the fiber drawn and produced has desired waveguide parameters, particularly cut-off wavelength.

It is also understood from the foregoing description that to have a fiber having cut-off wavelength within desired and controlled range, there is a need to substantially avoid change in fiber tension during entire process of drawing a fiber based on change in temperature of surface of that part of the preform which is outside the furnace.

With above aim to overcome problems, disadvantages and limitations of the prior art, the present inventors have surprisingly found that surface of that part of the preform which is outside the furnace, continuously dissipates heat before entering completely in the furnace meaning thereby there is continuous heat loss from the preform, and hence, of heating capacity of the heating means of furnace through surface of that part of the preform which is outside the furnace, which in-turn has been found to result in change [drop] in temperature of the preform inside the furnace, and therefore, change [increase] in fiber tension meaning thereby causes continuous change in values of waveguide parameters, particularly in desired value or range of cut-off wavelength.

Therefore, even if an attempt is made to control and maintain fiber tension of the fiber drawn by measuring fiber tension by a system described with reference to accompanying Figure 1, one cannot achieve substantial decrease in change of the fiber tension, because this action is done only after damage has actually been caused. According to this finding of present inventors, the system of Figure 1 which controls and maintains fiber tension based on continuous measurement of fiber tension only after the fiber has been drawn is not suitable for substantial decrease in change of the fiber tension. The present invention is a step towards overcoming this problem of the system described with reference to Figure 1.

The present inventors have also observed that with continuous movement of preform inside the furnace the surface area of preform which is available for heat losses continuously decreases which in-turn causes increase in surface temperature of that part of the preform which is still outside the furnace, because over a period of time the heat loss takes place through reduced surface area of the preform. According to this finding of present inventors, the spectroscopic means of JP 63123832 placed in direct contact

with and on top end of the preform gets damaged. The present invention is also a step towards overcoming this problem of the system taught in JP 63123832.

It has been further observed that none of the systems taught in prior art are suitable for controlling and maintaining the sudden changes in fiber tension of the fiber being drawn when the preform completely enters the furnace and causes sudden change of preform inside the furnace. The present invention is also a step towards overcoming this problem of the system taught in prior art.

Therefore, the present invention provides an apparatus and method for drawing a fiber wherein temperature of preform inside the furnace is controlled and maintained at a predetermined value or within predetermined range based on change in temperature of surface of that part of the preform which is outside the furnace, that is before the fiber has actually been drawn which has been found to have advantage of substantially controlling and maintaining the fiber tension during entire drawing process and of producing a fiber having desired waveguide parameters, particularly the cut-off wavelength, and therefore, to overcome above-described problems and drawbacks of the prior art. Further, it has also been found to advantage of substantially avoiding sudden change in fiber tension by controlling sudden variation in temperature of the preform when it completely enters the furnace, and therefore, change in fiber tension is substantially avoided during the entire process of drawing of a fiber and that's too before the fiber has been actually drawn.

Further, the presently disclosed apparatus has been advantageously found to be suitable for coupling with systems of prior art particularly the one described with reference to accompanying Figure 1 so that if desired, the value of fiber tension measured from initially drawn fiber can be employed as reference to control and maintain the fiber tension during the entire drawing process.

Accordingly, the present invention relates to an apparatus [Figure 2] for drawing a fiber having desired waveguide parameters, particularly cut-off wavelength comprising a furnace 101 having a furnace chamber 102 provided with heating means 103 having heating elements 104 for heating a preform 105 suitably suspended through preform inlet 106 inside the furnace 102 via its top end 107 from a suspension means 108 with the help of a handle rod 109 in a manner that its bottom end 110 coincides with center of the heating means 103

so that the preform tip 111 is suitably heated to a temperature suitable for drawing of a fiber 112, characterized in that:- a temperature measurement means 113 is provided near surface A of the preform 105, which is capable of directly measuring surface temperature of that part of the preform 105 which is outside the furnace; and a programmable logic controller [PLC] 114 connectable to said temperature measurement means 113, and capable of controlling and maintaining temperature of that part of the preform 105 which is inside the furnace 101 by continuously controlling and maintaining the power supply to heating elements 104 of the furnace, wherein the PLC 114 is capable of controlling and maintaining power supply to the heating elements 104 at a predetermined value or within predetermined range via controlled and maintained power supply to heating means 103 of the furnace 101 by employing equation (1) till the preform completely just enters the furnace:-

Power supply = (start power) x C + (surface temperature of preform which is outside furnace) x (slope term) Eqn. (1)

wherein, Power supply is the controlled and maintained power supplied to the heating elements 104 of the apparatus in accordance with present invention;

C is constant having value varying from 1.05 to 1.1;

Slope term is constant having value varying from -0.010 to -0.025; Start power is power of heating means measured at the time of beginning of the drawing process; and

Surface temperature of preform which is outside furnace is a temperature measured at point A of the preform with temperature measuring means 113; and by employing equation (2) after the preform has completely entered in the furnace:- '

Power supply = slope x ((π(d/2) ² χL _r )/ 1000) + Cl Eqn. (2)

wherein, Power supply is the controlled and maintained power supplied to the heating elements 104 of the apparatus in accordance with present invention; slope is a constant having value varying from 0.01 to 0.2;

Cl is constant having value varying from 35 to 55; d is the diameter of the preform measured along the length of the preform before start of drawing process;

Lr is the length of the preform inside the furnace measured with the help of PLC.

In one embodiment the present invention relates to an apparatus for drawing a fiber having desired waveguide parameters, particularly cut-off wavelength characterized in that:- a temperature measurement means 113 is provided near surface A of the preform 105, which is capable of directly measuring surface temperature of that part of the preform 105 which is outside the furnace; and a programmable logic controller [PLC] 114 connectable to said temperature measurement means 113, and capable of controlling and maintaining temperature of that part of the preform 105 which is inside the furnace 101 by continuously controlling and maintaining the power supply to heating elements 104 of the furnace, wherein the PLC 114 is capable of controlling and maintaining power supply to the heating elements 104 at a predetermined value or within predetermined range via controlled and maintained power supply to heating means 103 of the furnace 101 by employing equation (1) till the preform completely just enters the furnace :-

Power supply = (start power) x C + (surface temperature of preform which is outside furnace) x (slope term) Eqn. (1)

wherein, Power supply is the controlled and maintained power supplied to the heating elements 104 of the apparatus in accordance with present invention;

C is constant having value varying from 1.05 to 1.1; Slope term is constant having value varying from -0.010 to -0.025;

Start power is power of heating means measured at the time of beginning of the drawing process; and

Surface temperature of preform which is outside furnace is a temperature measured at point A of the preform with temperature measuring means 113.

In another embodiment the present invention relates to an apparatus for drawing a fiber having desired waveguide parameters, particularly cut-off wavelength characterized in that:- - a temperature measurement means 113 is provided near surface A of the preform 105, which is capable of directly measuring surface temperature of that part of the preform 105 which is outside the furnace; and a programmable logic controller [PLC] 114 connectable to said temperature measurement means 113, and capable of controlling and maintaining temperature of that part of the preform 105 which is inside the furnace 101 by continuously controlling and maintaining the power supply to heating elements 104 of the furnace, wherein the PLC 114 is capable of controlling and maintaining power supply to the heating elements 104 at a predetermined value or within predetermined range via controlled and maintained power supply to heating means 103 of the furnace 101 by employing equation (2) after the preform has completely entered in the furnace :-

Power supply = slope x ((π(d/2) ² xL _r )/ 1000) + Cl Eqn. (2)

wherein, Power supply is the controlled and maintained power supplied to the heating elements 104 of the apparatus in accordance with present invention; slope is a constant having value varying from 0.01 to 0.2;

Cl is constant having value varying from 35 to 55; d is the diameter of the preform measured along the length of the preform before start of drawing process;

L _x is the length of the preform inside the furnace measured with the help of PLC.

In accordance with present invention, the temperature measurement means 113 provided near surface A of the preform 105 has been found to have advantage of avoiding its heating due to radiations radiated by the preform. In accordance with present invention, the temperature measurement means is no-contact means provided with laser means to measure the temperature of the surface of the preform, and hence, is capable of being placed at a suitable distance from the surface of the preform, which further avoids its heating due to radiations radiated by the preform. In accordance with one of the preferred embodiments of the present invention, the temperature measurement means 113 for measuring surface temperature of the preform is provided adjacent to preform surface marked as A and preform inlet 106, preferably above the diffuser plate 115 of the diffuser 116 provided on top of the furnace 101, which has been found to have advantage of substantially avoiding its heating due to heat losses from that part of the preform which is outside the furnace.

In accordance with present invention, the temperature measurement means 113 is optionally provided adjacent to preform surface marked as A and preform inlet 106 with the help of a mounting means [not shown in figures] which has been found to have advantage of avoiding change in its distance visa-vis preform.

In accordance with present invention, the PLC is capable of controlling and maintaining temperature of preform 105 inside the furnace 101 by controlling and maintaining power supply to the heating elements 104 at a predetermined value or within predetermined range via controlled and maintained power supply to heating means 103 of the furnace 101 by employing above equation 1 based on surface temperature of that part of preform which is outside furnace and measured with temperature measuring means 113 of the present apparatus till the preform completely enters the furnace, which as described herein, has been found to overcome problems and drawbacks of the prior art.

In accordance with present invention, the PLC is also capable of capable of controlling and maintaining temperature of preform 105 inside the furnace 101 by controlling and maintaining power supply to the heating elements 104 at a predetermined value or within predetermined range via controlled and

maintained power supply to heating means 103 of the furnace 101 by employing above equation 2 based on diameter [d] of the preform measured along its length before the start of drawing process and length of the preform inside the furnace [L _r ] measured by PLC of the present apparatus after the preform has completely entered in the furnace, which as described herein, has been found to overcome problems and drawbacks of the prior art particularly the sudden variations in the fiber tension of drawn fiber on complete insertion of the preform inside the furnace. It is, therefore, understood that present apparatus has been found to have advantage of overcoming above-described problems and drawbacks of the prior art during the entire drawing process.

In accordance with present invention, the diameter [d] of the preform can be measured along the length of the preform before start of drawing process by any conventional means known in the art.

In one embodiment, the present invention also relates to a method for drawing a fiber having desired waveguide parameters, particularly the cut-off wavelength comprising continuously measuring surface temperature of that part of preform which is outside the furnace [for illustrations in accompanying Figure 2 marked as surface A of the preform 105] by temperature measurement means 113 suitably provided in the presently disclosed apparatus for drawing a fiber 112 and feeding the measured temperature of surface A of the preform to a programmable logic controller 114 of present apparatus to control and maintain temperature of preform 105 inside the furnace 101 by controlling and maintaining power supply to the heating elements 104 via controlling and maintaining power supply to heating means 103 of the furnace 101 by employing equation 1:- Poiυer supply = (start power) x C + (surface temperature of preform which is outside furnace) x (slope term) Eqn. (1)

wherein, Power supply is the controlled and maintained power supplied to the heating elements 104 of the apparatus in accordance with present invention;

C is constant having value varying from 1.05 to 1.1; Slope term is constant having value varying from -0.010 to -0.025;

Start power is power of heating means measured at the time of beginning of the drawing process; and

Surface temperature of preform which is outside furnace is a temperature measured at , point A of the preform with temperature measuring means 113;

In another embodiment, the method of drawing fiber in accordance with present invention comprises continuously measuring length of the preform inside the furnace [Lr] by PLC of the present apparatus after the preform has completely entered in the furnace, and controlling and maintaining temperature of preform 105 inside the furnace 101 by controlling and maintaining power supply to the heating elements 104 at a predetermined value or within predetermined range via controlled and maintained power supply to heating means 103 of the furnace 101 by employing equation 2 based on diameter [d] of the preform measured, before the start of drawing process, along length of the preform inside the furnace and length [Lr] of the preform inside the furnace measured by PLC of the present apparatus after the preform has completely entered in the furnace: -

Power supply = slope x ((π(d/2) ² χLr)/ 1000) + Cl Eqn. (2)

wherein, Power supply is the controlled and maintained power supplied - to the heating elements 104 of the apparatus in accordance with present invention; slope is a constant having value varying from 0.01 to 0.2; Cl is constant having value varying from 35 to 55; d is the diameter of the preform measured along the length of the preform before start of drawing process;

Lr is the length of the preform inside the furnace measured with the help of PLC. As described herein, the method of present invention when carried out by employing apparatus of present invention has been found to overcome problems and drawbacks of the prior art.

In accordance with one of the embodiments of the present invention, the surface temperature of surface A of the preform 105 is measured continuously and at a regular interval. In accordance with another embodiment of the present invention, the fiber tension is initially measured with tensionometer 117 provided suitably at or near the furnace outlet 118 and measured values is employed as reference for maintaining the fiber tension of the fiber being drawn thereafter by employing apparatus and method present invention. In accordance with preferred embodiment of the present invention, any variation in fiber tension initially measured at start of fiber drawing process and used as reference value through out the entire fiber drawing process is substantially avoided when fiber is drawn in accordance with method of the present invention while employing presently disclosed apparatus for drawing of a fiber. Therefore, the present invention has advantage that the fiber tension is measured only once in beginning of the process.

Accordingly, an apparatus and method for drawing a fiber has been provided wherein heat losses [as shown by arrows 119 in Figure 2] from surface [A] of that part of the preform 105 which is outside the furnace guides control and maintenance of temperature of preform 105 inside the furnace 101 when such heat loss 119 is measured by temperature measurement means 103 as surface temperature of the preform 105 and fed to controller 114 for controlling and maintaining the power supply of heating means 103 which in- turn controls and maintains temperature of the preform 105 inside the furnace 101, and diameter of the preform measured at beginning of drawing process and length of the preform inside the furnace continuously measured by PLC guides control and maintenance of temperature of preform 105 inside the furnace 101 by employing controller 114 which is capable of controlling and maintaining the power supply of heating means 103 which in- turn controls and maintains temperature of the preform 105 inside the furnace 101.

The present invention is now described with the help of following examples which not intended to limit scope of this invention, but are incorporated merely to illustrate the manner of performing the present invention. Examples:

Example 1 (prior art):

A preform of 70 cm length and average diameter of about 84.20 mm was suspended in a fiber drawing furnace. The preform was heated to soften the tip and the fiber drawing was started. The tension in the fiber was to be kept at 70 g to have the desired waveguide parameter, particularly cut-off wavelength. The tension in the fiber was measured using tensionometer and the preform temperature was controlled using the fiber tension value measured using the tensionometer by employing apparatus as illustrated in accompanying Figure 1. It was observed that the fiber tension could not be controlled precisely during the complete drawing process due to the reasons described herein primarily due to the fact it is controlled and maintained based on fiber tension measured on drawn fiber. A variation of about ±20 g [about + 28.575% variation was observed] in the fiber tension from the desired value of about 70 g waβ observed during the drawing of the preform till the preform completely enters in the furnace. When the preform completely enters the furnace, it was surprisingly observed that by employing apparatus illustrated in Figure 1 , the tension in the fiber could not be controlled and it suddenly dropped from about 70 g to about 50 g [about +, 28.575% variation was observed] and then went on reducing to about 30 g [about + 57.142 % variation was observed]. Under the circumstance, even on employing the apparatus of Figure 1 having PLC means, the tension had to be controlled manually by reducing the furnace temperature to reduce the preform temperature. Further, the cut-off wavelength parameter of the drawn fiber was found to be out of range, i.e. 1150 nm to 1170 nm, which was particularly further reduced/increased to about 1130 nm to 1160 nm for the part of fiber drawn at the end of the preform when it had completely entered the furnace.

Therefore, the apparatus and method of prior art suffers from above- described problems and drawbacks. Example 2 (present invention):

A preform of 70 cm length and average diameter of about 84.51 mm was suspended in a fiber drawing furnace. The diameter of the preform was measured along its entire length before the start of drawing process and fed to PLC of apparatus of present invention as illustrated by accompanying Figure 2. The preform was heated to soften the tip and the fiber drawing was started.

Simultaneously, the surface temperature of that part of the preform which was outside the furnace was measured with the help of non-contact temperature measurement means of the present invention and continuously fed to PLC. The tension in the fiber was to be kept at 7O g to have the desired waveguide parameter, particularly the cut-off wavelength. The tension in the fiber was measured only once using tensionometer and was merely employed as reference. The PLC of present apparatus controlled and maintained the preform temperature 2090 ± 10 ⁰ C by employing above-described equation 1 till the preform entered the furnace [Figure 3]. Once the preform completely entered the furnace, the PLC of present apparatus controlled and maintained the preform temperature 2085 ± 1O ⁰ C by employing above-described equation 2. The surface temperature of the preform is illustrated by graph 501 of accompanying Figure 4(b), wherein the surface temperature increases with decrease in preform length remaining outside the furnace. The PLC employs equation 1 till the surface temperature reaches point 502 on graph 501, which represents the point when the preform has just completely entered the furnace and thereafter employs equation 2 when the preform has completed entered the furnace to control and maintain the power supply to heating elements. The fiber tension measured illustrates graph 403 of accompanying Figure 4(a) when plotted which was found to be very close to the ideal graph 402 of Figure 4(a).

Therefore, it was observed that the fiber tension could be substantially controlled and maintained very precisely during the complete drawing process due to the technical effects of present invention described herein primarily due to the fact it is controlled and maintained based on surface temperature of that part of preform which is outside the furnace. A variation of about + 2 g [about + 2.857% variation was observed] in the fiber tension from the desired value of about 70 g was observed during the drawing of the preform till the preform completely enters in the furnace. When the preform completely enters the furnace, it was surprisingly observed that by employing apparatus present invention as illustrated in Figure 2 and Figure 3, the tension in the fiber could also be substantially controlled and maintained it did not drop from about 70 g to about 65 g [about + 7.14% variation was observed] and then was kept almost constant and reduced only to about 68 g [about -2.85 % variation was observed]. Under the circumstance, there was no need to have manual control

of preform temperature inside the furnace to manually control the fiber tension. Further, the cut-off wavelength parameter of the drawn fiber was found to be well within the range, i.e. about 1230 run to 1240 nm, which was particularly was also maintained further at about 1230 nm to 1240 nm for the part of fiber drawn at the end of the preform when it had completely entered the furnace.

Table 1 illustrates the power supply calculated by employing equation 1 and Table 2 illustrates the power supply calculated by employing equation 2.

Table 1

It was thus observed that the preform temperature inside the furnace, and hence, the fiber tension could be substantially controlled and maintained by employing apparatus and method present invention without requiring need of manual intervention for controlling the temperature and hence the fiber tension.

It may be noted that various terms as employed herein are merely intended to illustrate the present invention and are not intended to restrict scope of the present invention. It is obvious for the persons skilled in the art that alternative terms may also be employed to describe the present apparatus and method without deviating from the intended scope of the present invention.

It may also be noted that the presently disclosed apparatus and method have been described with reference to commonly employed method of drawing optical fiber. However, the present apparatus and method are also suitable even for other alternative methods for drawing fiber. Therefore, in one embodiment such modifications are included within the scope of present invention.