METHOD FOR CONTROLLING SURFACE DISTANCE IN LONGITUDINAL DIRECTION OF PREFORM IN MANUFACTURING PROCESS OF OPTICAL FIBER PREFORM, AND

SYSTEM FOR IMPLEMENTING THE METHOD Technical Field

[1] The present invention relates to manufacture of an optical fiber preform, and more particularly a method for controlling a surface distance (e.g., an outer diameter or an inner diameter) of a preform uniformly during an optical fiber preform manufacturing process and a system for implementing the method. Background Art

[2] In a conventional optical fiber preform manufacturing method using MCVD

(Modified Chemical Vapor Deposition), a rotating quartz tube is heated to a high temperature using a heat source such as an oxygen-hydrogen torch, a plasma heater or an electric furnace, which reciprocates along a process progressing direction, and at the same time a soot generation and an oxidization circumstance making gas are injected into the quartz tube together with a carrier gas.

[3] Then, particulate soot is generated due to oxidization reaction of the soot generation gas, and this soot is seated on an inner surface of the quartz tube positioned in front of the heat source by means of thermophoresis, and then sintered by a subsequently approaching heat source to form a clad layer or a core layer. This process is repeated for each layer so that a clad and a core are formed in the quartz tube to have desired thicknesses, and then they are collapsed to complete an optical fiber preform.

[4] Optical signals are transmitted through a core of the optical fiber, so a refractive index profile (a refractive index of the core > a refractive index of the clad) is endowed in a radial direction of the preform when an optical fiber preform is manufactured. Accordingly, refractive indexes of materials deposited in each layer using MCVD are not identical to each other but slightly different depending on the refractive index profile.

[5] In this reason, if a deposition amount in an axial direction of the quartz tube is not constant in the unit layer deposition process, the refractive index profile is distorted in an axial direction of the optical fiber preform. That is to say, diffractive indexes have different values in spite of the same radial position based on the center axis of the quartz tube. This refractive index profile deteriorates an optical characteristic of the optical fiber manufactured from an optical fiber preform, as well known in the art.

[6] Thus, one of important purposes in the repeated layer-based depositing process of a

clad layer or a core layer using MCVD is that a previously scheduled deposition amount is uniformly deposited in each layer along an axial direction of the quartz tube so that all points having the same radius based on the center axis of the preform have the same refractive index.

[7] For this purpose, when the laser-based depositing process is progressed, process conditions of MCVD should be precisely controlled so that an inner diameter of the quartz tube, namely an inner diameter of the preform is kept uniformly. However, in the prior art, there is no suggested technique for measuring and monitoring an inner diameter of the preform while the MCVD is progressed, so a method for indirectly controlling an inner diameter of the preform by controlling an outer diameter of the preform is frequently used as an alternative.

[8] An outer diameter controller adopted in MCVD is generally designed in consideration of the above characteristics of MCVD. That is to say, a target value of the preform outer diameter is set in advance when the layer-based deposition process is progressed, and a preform outer diameter at a hot zone of the heat source is compared with the target outer diameter with the heat source being moved in a process advancing direction. If the measured preform outer diameter is different from the target outer diameter, a control input such as an inner pressure of the quartz tube is changed in real time by means of feedback control so that the preform outer diameter is rapidly changed close to the target outer diameter, as shown in FIG. 1.

[9] However, since MCVD gives a very slow output response in which a 'time delay' is over several seconds and a 'processing constant' is also over about several seconds or several ten seconds, a preform outer diameter measured at the hot zone cannot be considered as an exact outer diameter that reflects 100% of an output response according to the change of a control input. Here, the term 'time delay' is defined as a time T taken from that the control input is changed at a predetermined time point t as shown in FIG. 2 till the preform outer diameter starts changing, and the term 'processing constant' is defined as a time T taken from that the control input is changed as shown in FIG. 2 till a changing amount of the preform outer diameter reaches 63%, assuming that a final changing amount of the preform outer diameter is 100%.

[10] Thus, in case the outer diameter controller is designed so that a preform outer diameter rapidly reaches the target outer diameter in MCVD, the preform outer diameter is rapidly changed to reach the target outer diameter in an actual case as shown in FIG. 3, thereby not ensuring proper outer diameter control. In order to solve this problem, the outer diameter controller may be designed so that a preform outer diameter reaches the target outer diameter within a sufficient amount of time. However, in this case, a time for the preform outer diameter to reach the target outer

diameter is extended and thus the preform outer diameter does not chase the target outer diameter in real time, thereby deteriorating the performance of the outer diameter controller.

[11] In addition, the conventional outer diameter controller uses a real-time feedback control algorithm, so this conventional controller requires a controlling expert having a professional knowledge and consumes much time and cost for designing the control algorithm.

Disclosure of Invention

Technical Problem

[12] The present invention is designed in consideration of the above problems, and therefore it is an object of the invention to provide a method for controlling a surface distance of an optical fiber preform, which may prevent distortion of a refractive index profile by keeping the surface distance (e.g., an inner diameter or an outer diameter) of the preform uniformly in an axial direction by means of the control based on a static response characteristic of a unit process, not by making a control response to trace a target response rapidly by means of a real-time feedback control, during an optical fiber preform manufacturing process characterized in repetition of unit processes, and also to provide a system for implementing the system. Technical Solution

[13] In order to accomplish the above object, the present invention provides a method for controlling a distance from a center axis of a preform to a surface of the preform uniformly along an axial direction while a material film composing an optical fiber is repeatedly deposited layer by layer on a preform deposition member so as to make an optical fiber preform, the method including (a) measuring surface distances at a plurality of control response completion points where a control input is already applied and thus the surface distance is substantially not changed; (b) approximating a surface distance at a control input applying point where a control input will be applied using the measured surface distance; (c) calculating a control input for reducing a difference between a control target surface distance at the control input applying point and the surface distance approximated in the step (b) by using a static response characteristic at the control response completion point; and (d) applying the calculated control input to the control input applying point.

[14] Preferably, the preform deposition member is a preform tube having an inner hollow. In this case, the optical fiber material film is repeatedly deposited on an inner surface of the preform tube by means of MCVD (Modified Chemical Vapor Deposition).

[15] In addition, the surface distance is a preform inner diameter corresponding to a

distance between inner circumferences based on a central axis of the preform tube, or a preform outer diameter corresponding to a distance between outer circumferences based on the central axis of the preform tube. [16] According to one embodiment of the present invention, in the step (a), surface distances for a plurality of control response completion points are measured in a n- 1 layer deposition process by using a predetermined surface distance measurer, in the step (b), a surface distance profile for a plurality of control input applying points is approximated in a n layer deposition process by using a measured surface distance data group, and, in the step (c), a control input for each control input applying point in the n th layer deposition process is calculated using a static response characteristic value obtained for the n- 1 layer deposition process, a control target surface distance of n layer deposition process, and the surface distance profile.

[17] As one embodiment, the surface distance measurer is installed at the rear of a heat source so as to be carried together with the heat source. In this case, in the step (a), surface distances for a plurality of control response completion points are measured at the rear of the heat source by using the surface distance measurer when the heat source is carried in a process advancing direction.

[18] As another embodiment, the surface distance measurer is installed at a center of a heat source so as to be carried together with the heat source. In this case, in the step (a), surface distances for a plurality of control response completion points are measured at the center of the heat source by using the surface distance measurer while the heat source is returned to a process start point.

[19] As still another embodiment of the present invention, the surface distance measurer is loaded on a carrying means separately from a heat source. In this case, in the step (a), the surface distance measurer measures surface distances for a plurality of control response completion points with being separately carried at the rear of the heat source to be spaced apart from the heat source by a predetermined distance while the heat source is carried to a process start point.

[20] In the present invention, in the step (b), a surface distance at the control input applying point is approximated using a surface distance data measured at two or more control response completion points that approach the control input applying point.

[21] Preferably, the static response characteristic is numerically expressed using: a value averaging a ratio of a final surface distance changing amount to a control input changing amount at every control response completion point; a value selected from a ratio of a final surface distance changing amount to a control input changing amount at every control response completion point; or a value averaging the predetermined number of ratios selected from a ratio of a final surface distance changing amount to a control input changing amount at every control response completion point. In addition,

a control input at each control input applying point is calculated by multiplying a difference between a control target surface distance and an approximated surface distance at the corresponding point by the static response characteristic and a predetermined proportional factor, and then adding it with a criterion control input.

[22] Preferably, the control input is a pressure in a region where a deposition process is conducted, a temperature of a heat source, a moving velocity of the heating source, or their selective combination.

[23] According to another embodiment of the present invention, in the step (a), while a heat source is carried in a process advancing direction in a n layer deposition process, surface distances for a plurality of control response completion points to which a control input is applied in a n-l' layer deposition process, by using a surface distance measurer spaced apart from the heat source in front of the heat source by a predetermined distance and carried together with the heat source, in the step (b), in parallel to the step (a), a surface distance at a control input applying point is approximated using a surface distance data measured at an approaching control response completion point before the heat source passes each control input applying point in the n' layer deposition process, and, in the step (c), after the step (b), a control input for the control input applying point is calculated using the already measured static response characteristic at the control response completion point, a control target surface distance of the n layer deposition process, and the approximated surface distance.

[24] Preferably, the static response characteristic is numerically expressed using: a value averaging a ratio of a final surface distance changing amount to a control input changing amount of the n- 1 layer deposition process at every control response completion point whose surface distance is already measured; a value selected from a ratio of a final surface distance changing amount to a control input changing amount of the n-l layer deposition process at every control response completion point whose surface distance is already measured; or a value averaging the predetermined number of ratios selected from a ratio of a final surface distance changing amount to a control input changing amount of the n- 1 layer deposition process at every control response completion point whose surface distance is already measured. In addition, a control input at each control input applying point is calculated by multiplying a difference between a control target surface distance and an approximated surface distance at the corresponding point by the static response characteristic and a predetermined proportional factor, and then adding it with a criterion control input.

[25] In another aspect of the invention to accomplish the above object, there is provided a method for controlling a distance from a center axis of a preform tube to a surface of the preform uniformly along an axial direction while a hollow in the preform tube is

removed by reciprocating a heat along the axial direction of the preform tube in which a deposition process of an optical fiber material film is completed, the method including (a) measuring surface distances at a plurality of control response completion points where a control input is already applied and thus the surface distance is substantially not changed; (b) approximating a surface distance at a control input applying point where a control input will be applied using the measured surface distance; (c) calculating a control input for reducing a difference between a control target surface distance at the control input applying point and the surface distance approximated in the step (b) by using a static response characteristic at the control response completion point; and (d) applying the calculated control input to the control input applying point.

[26] In still another aspect of the invention to accomplish the above object, there is also provided a system for controlling a distance from a center axis of a preform to a surface of the preform uniformly along an axial direction while a material film composing an optical fiber is repeatedly deposited layer by layer so as to make an optical fiber preform, the system including: (a) a preform deposition member on which the optical fiber material film is repeatedly deposited; (b) a heat source reciprocated along an axial direction of the preform deposition member and heating the preform deposition member to a temperature over an optical fiber material film generating temperature; (c) a control input controller for applying a control input of a process related to a deposition amount when the heat source passes a control input applying point, in the layer deposition process of the optical fiber material film; (d) a surface distance measurer for measuring surface distances at a plurality of control response completion points where the control input is already applied and thus the surface distance is substantially not changed; and (e) a surface distance controller for approximating a surface distance at a control input applying point to which the control input will be applied with the use of the measured surface distance received from the surface distance measurer, calculating a control input using a static response characteristic at the control input completion point in order to reduce a difference between the approximated surface distance and a control target surface distance at the control input applying point, and applying the calculated control input to the control input applying point by controlling the control input controller.

[27] As one embodiment, the surface distance measurer is installed at the rear of the heat source and carried together with the heat source at a position spaced apart from the heat source, or loaded on a separate carrying means and follows the heat source at a rear position thereof with keeping a predetermined distance from the heat source when the heat source is carried in a process advancing direction. The surface distance measurer measures surface distances at a plurality of control response completion points while the heat source is carried in a process advancing direction in a n- 1 layer

deposition process, while the heat source is returned to a process start point in the n-1 layer deposition process, or while the heat source is carried again in a process advancing direction without application of a control input after the heat source is returned to the process start point in the n-1 layer deposition process. Before a n layer deposition process starts, the surface distance controller approximates an outer diameter profile for a control input applying point in the n layer deposition process, calculates a static response characteristic value for the n- 1 layer deposition process by using the measured surface distance, and calculates a n control input by multiplying a difference between a n control target surface distance and an approximated surface distance at each control input applying point by the static response characteristic and a predetermined proportional factor and then adding a n criterion control input thereto.

[28] As another embodiment, the surface distance measurer is installed at a center of the heat source and carried together with the heat source. The surface distance measurer measures surface distances at a plurality of control response completion points while the heat source is returned to a process start point in a n-1 layer deposition process, or while the heat source is carried again in a process advancing direction without application of a control input after the heat source is returned to the process start point in the n-1 layer deposition process. Before a n layer deposition process starts, the surface distance controller approximates an outer diameter profile for a control input applying point in the n layer deposition process, calculates a static response characteristic value for the n-1 layer deposition process by using the measured surface distance, and calculates a n' control input by multiplying a difference between a n control target surface distance and an approximated surface distance at each control input applying point by the static response characteristic and a predetermined proportional factor and then adding a n criterion control input thereto.

[29] As another embodiment, the surface distance measurer is loaded on a separate carrying means. The surface distance measurer measures surface distances at a plurality of control response completion points with moving in the process advancing direction after a n-l' layer deposition process is completed. Before a n layer deposition process starts, the surface distance controller approximates an outer diameter profile for a control input applying point in the n layer deposition process, calculates a static response characteristic value for the n- 1 layer deposition process by using the measured surface distance, and calculates a n control input by multiplying a difference between a n control target surface distance and an approximated surface distance at each control input applying point by the static response characteristic and a predetermined proportional factor and then adding a n criterion control input thereto.

[30] As further another embodiment, the surface distance measurer is installed in front of the heat source and carried together with the heat source at a position spaced apart

from the heat source. The surface distance measurer measures surface distances at a plurality of control response completion points while the heat source is carried in a process advancing direction in a n layer deposition process. Before the heat source passes each control input applying point of the n layer deposition process, the surface distance controller approximates a surface distance at a control input applying point by using the measured surface distance data at a control response completion point that approaches before the heat source passes each control input applying point in the n layer deposition process, calculates a static response characteristic value at the control response completion point whose surface distance is measured, calculates a control input by multiplying a difference between a n control target surface distance and the approximated surface distance at each control input applying point by a predetermined proportional factor and the static response characteristic value and then adding a criterion control input thereto, and applying the control input when the heat passes the control input applying point.

[31] As still another embodiment, the surface distance measurer is loaded on a separate carrying means and leads the heat source with keeping a predetermined distance in front of the heat source when the heat source is carried in a process advancing direction. The surface distance measurer measures surface distances at a plurality of control response completion points in front of the heat source while the heat source is carried in the process advancing direction during a n layer deposition process. Before the heat source passes each control input applying point of the n layer deposition process, the surface distance controller approximates a surface distance at a control input applying point by using the measured surface distance data at a control response completion point that approaches before the heat source passes each control input applying point in the n layer deposition process, calculates a static response characteristic value at the control response completion point whose surface distance is measured, calculates a control input by multiplying a difference between a n control target surface distance and the approximated surface distance at each control input applying point by a predetermined proportional factor and the static response characteristic value and then adding a criterion control input thereto, and applying the control input when the heat passes the control input applying point.

[32] In further another embodiment, first and second surface distance measurers are respectively installed at the front and rear of the heat source and carried together with the heat source. The second surface distance measurer measures surface distances at a plurality of control response completion points from a process start point to a first point while the heat source is carried in a process advancing direction in a n-1 layer deposition process. The first surface distance measurer measures surface distances at a plurality of control response completion points from the first point to a process end

point while the heat source is carried in a process advancing direction in a n-1 layer deposition process. Before the heat source passes each control input applying point of the n layer deposition process, the surface distance controller approximates a surface distance at a control input applying point by using the measured surface distance data at a control response completion point that approaches before the heat source passes each control input applying point in the n layer deposition process, calculates a static response characteristic value at the control response completion point whose surface distance is measured, calculates a control input by multiplying a difference between a n control target surface distance and the approximated surface distance at each control input applying point by a predetermined proportional factor and the static response characteristic value and then adding a criterion control input thereto, and applying the control input when the heat passes the control input applying point.

[33] Preferably, the preform deposition member is a preform tube having an inner hollow so that the optical fiber material film is repeatedly deposited on an inner surface thereof. In this case, the surface distance is a preform inner diameter corresponding to a distance between inner circumferences based on a central axis of the preform tube, or a preform outer diameter corresponding to a distance between outer circumferences based on the central axis of the preform tube.

[34] As an alternative, the preform deposition member is a preform rod on an outer surface of which the optical fiber material film is repeatedly deposited. In this case, the surface is a preform outer diameter corresponding to a distance between outer circumferences based on a center axis of the preform rod. Brief Description of the Drawings

[35] These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings:

[36] FIG. 1 is a graph showing that a preform surface distance is changed depending on the change of a step control input in an optical fiber preform manufacturing process using MCVD;

[37] FIG. 2 is a graph showing concepts of a time delay, a processing constant and a process stabilizing time;

[38] FIG. 3 is a graph showing that an outer diameter of a preform is rapidly changed when the outer diameter of the preform is controlled using a conventional real-time feedback control;

[39] FIG. 4 is a schematic view showing a surface distance control system of an optical fiber preform according to an embodiment of the present invention;

[40] FIG. 5 is a sectional view showing an optical fiber preform, which illustrates a

concept of a preform surface distance; [41] FIG. 6 is a schematic view showing a surface distance control program of a preform according to an embodiment of the present invention; [42] FIG. 7 is a graph illustrating a method for approximating an outer diameter at a control input applying point by using the first order interpolation in a n layer deposition process; [43] FIG. 8 is a detailed flowchart illustrating a method for controlling a surface distance of a preform according to an embodiment of the present invention; [44] FIG. 9 shows an arrangement relation between a surface distance measurer and a heat source according to one embodiment of the present invention; [45] FIG. 10 shows an arrangement relation between a surface distance measurer and a heat source according to another embodiment of the present invention; [46] FIG. 11 shows an arrangement relation between a surface distance measurer and a heat source according to still another embodiment of the present invention; [47] FIG. 12 shows an arrangement relation between a surface distance measurer and a heat source according to further another embodiment of the present invention; [48] FIG. 13 is a schematic view showing the surface distance measurer;

[49] FIG. 14 is a sectional view showing a screen coated with a light diffuser, employed in the surface distance measurer shown in FIG. 13; [50] FIGs. 15 and 16 are perspective views showing light beam patterns looked on the screen when a preform deposition member to be measured is positioned or not positioned on a light path, respectively; [51] FIG. 17 shows a path looked as a light beam pattern on the screen since a linear light beam irradiated to the preform deposition member is hidden or refracted by the preform deposition member, and also a light beam pattern looked on the screen; [52] FIG. 18 is a diagram illustrating the law of refraction, which is observed when a light beam passes through different kinds of media having different refractive indexes; and [53] FIG. 19 is a diagram illustrating a process of extracting inner diameter information of the preform deposition member.

Best Mode for Carrying Out the Invention [54] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms ap-

propriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[55] FIG. 4 shows a system I for controlling an axial surface distance of a preform uniformly while an optical fiber preform is manufactured by repeatedly depositing an optical fiber material film on a preform deposition member 110 layer by layer using MCVD according to one embodiment of the present invention.

[56] Referring to FIG. 4, the surface distance control system I of this embodiment includes a preform deposition member 110, a heat source 120, a surface distance measurer 140, first to third control input controllers 170a, 170b, 170c, and a surface distance controller 150.

[57] The preform deposition member 110 adopts a quartz tube having an inner hollow, and optical fiber material films are repeatedly deposited on an inner circumference of the quartz tube by means of MCVD. A surface distance of the preform is a distance r between inner circumferences of the quartz tube (or, an inner diameter of the preform) based on a center axis O of the preform deposition member 110 as shown in FIG. 5, or a distance r between outer circumferences of the quartz tube (namely, an outer diameter of the preform) based on the center axis O of the preform deposition member 110.

[58] The heat source 120 for heating the preform deposition member 110 so that the optical fiber material film is deposited on the inner circumference thereof adopts a resistance-heating graphite electric furnace. However, the heat source 120 may adopt any heat source that may be suitably used in the optical fiber preform manufacturing process using MCVD, such as an induced-heating electric furnace, an oxygen- hydrogen torch or a plasma heat source, not limited to the above.

[59] The preform deposition member 110 is rotated based on its longitudinal axis, and a soot generation gas and an oxidization circumstance making gas are put into the preform deposition member 110 together with a carrier gas while the MCVD method is progressed. The soot generation gas uses a mixture gas of chemical gases pertaining to a halide group such as SiCl gas, POCl gas and GeCl gas, the oxidization circumstance making gas uses O gas, and the carrier gas uses an inert gas such as N gas, He gas or Ar gas.

[60] A gas supplying device 160 for supplying various gases required for manufacturing an optical fiber preform is coupled to one end of the preform deposition member 110, and a reaction byproduct discharging device 170 for discharging various reaction byproducts generated in the preform deposition member 110 is coupled to the other end of the preform deposition member 110. A gas supply conduit 180 is connected to

the gas supplying device 160 by means of a first control input controller 170a. The first control input controller 170a is a known flow rate controller and it controls a flow rate of chemical gas supplied through the gas supply conduit 180. The soot generation gas, the oxidization circumstance making gas and the carrier gas are injected through the gas supply conduit 180.

[61] The heat source 120 is installed to be reciprocating along an axial direction of the preform deposition member 110 with substantially surrounding an outer circumference of the preform deposition member 110. For this purpose, the heat source 120 is mounted to a rail 190 substantially parallel to the center axis of the preform deposition member 110, and it is reciprocated between a process start point and a process end point along the central axis of the preform deposition member 110 on the rail 190 by means of a heat source carrying means 195 such as a rail motor. A moving velocity and a direction of the heat source 120 is controlled by means of a second control input controller 170b.

[62] The heat source 120 controls a temperature using a third control input controller

170c. The third control input controller 170c regulates a power supplied to the heat source so as to control a caloric value of the heat source 120. The temperature controlling mechanism of the third control input controller 170c may be changed depending on the kind of the heat source 120. The heat source 120 gives a high temperature environment over 1600 ⁰ C so that oxidization reaction of the soot generation gas may be induced in the preform deposition member 110 positioned right above the heat source 120.

[63] The heat source 120 heats the preform deposition member 110 with moving in a process advancing direction (or, a direction from the right to the left on the drawing). At this time, the soot generation gas, the oxidization circumstance making gas and the carrier gas are put into the preform deposition member 110. Then, in the preform deposition member 110 right above the heat source 120, particulate soot is generated due to the oxidization reaction of the soot generation gas. In addition, the soot is moved toward an inner surface of the preform deposition member 110 in a region where the heat source 120 is already passed and it has a relatively lower temperature than the portion right above the heat source 120, and then seated thereon to form a soot layer of a predetermined thickness, by means of thermophoresis. The soot layer formed as mentioned above is sintered due to a high temperature given by a subsequently approaching heat source 120, thereby making an optical fiber material film of a unit layer.

[64] The heat source 120 is carried from the process start point to the process end point, and then returned. If the heat source 120 is returned, the unit layer deposition process is completed. This unit layer deposition process is repeated until clad and core are

deposited to desired thicknesses in the preform deposition member 110. In addition, a flow rate of the chemical gas supplied into the preform deposition tube 110 is controlled in correspondence to a refractive index profile of the optical fiber preform.

[65] The surface distance control system I of the present invention controls process conditions influenced on a deposition amount of the optical fiber material film, namely an inner pressure of the preform deposition member 110, a temperature of the heat source 120, a moving velocity of the heat source 120 or any of their combinations, while the unit layer deposition process is progressed so that a surface distance becomes uniform along the axial direction of the preform deposition member 110. Hereinafter, the above process conditions are called 'control input'.

[66] The surface distance measurer 140 is installed at the rear of the heat source 120, looked based on the process advancing direction of the heat source 120, in order to measure a surface distance at a control response completion point. As explained later, the surface distance measurer 140 may be designed to be at various positions. The surface distance measurer 140 is spaced apart from the heat source 120 by a predetermined distance, and carried together with the heat source 120 with keeping the spacing distance constantly.

[67] The surface distance measurer 140 measures surface distances at a plurality of control response completion points while the heat source 120 moves in the process advancing direction or returns to its original position, and it outputs each of the measured surface distance data to the surface distance controller 150.

[68] Here, the control response completion point is defined as follows. While the heat source 120 advances in the process advancing direction, if a control input such as an inner pressure of the preform deposition member 110 is changed when a hot zone of the heat source 120 passes a predetermined point of the preform deposition member 110, a surface distance such as a preform outer diameter is changed dependent on time as shown in FIG. 2. The control response completion point is defined as a point where there is no further change of surface distance since a process stabilizing time T is passed after the hot zone of the heat source 120 passes the point.

[69] The surface distance controller 150 collects surface distances of the preform for a plurality of control response completion points in a n- 1 (n>2) layer deposition process, and then calculates a n surface distance profile for a plurality of control input applying points whose control inputs will be applied in a n layer deposition process by means of a predetermined approximation method using the collected surface distances. Here, the surface distance profile is a group of surface distance data approximated for each control input applying point.

[70] Specifically, the surface distance controller 150 approximates a surface distance at a control input applying point by using surface distances measured at two or more

control response completion points that approach the control input applying point. Preferably, the above approximation method is first order interpolation. However, the present invention is not limited thereto.

[71] After that, the surface distance controller 150 schedules a n control input at each control input applying points, which may compensate a difference between the n surface distance profile and a control target surface distance of the n layer deposition process. At this time, a static response characteristic of the n-1 layer deposition process is considered. And then, in the n layer deposition process, the corresponding scheduled control input is applied whenever the heat source 120 passes each control input applying point. If the number of control input applying points is p, the n' control input is a group of the p number of control inputs.

[72] In the present invention, the static response characteristic is defined as a process response characteristic in the n-l' layer deposition process after a process stabilizing time at each control input applying point is passed. The static response characteristic at each control input applying point may be numerically calculated as

G; using the following equation 1. [73]

[74] Equation 1

[75]

«_ Changing amount of Final surface distance

Gr ■

Changing amount of Control input

[76]

[77] Here, a control input changing amount (the dominator) is a difference between a predetermined criterion control input and an actually applied control input at a j' control input applying point in the n-l layer deposition process. In addition, the final surface distance changing amount (the numerator) is a difference between a surface distance before the control input is applied and a surface distance when the control response is completed since a process stabilizing time is passed after the control input is changed at the j control input applying point in the n-l layer deposition process.

[78] Preferably, in order to schedule the n control input, an average value of static response characteristic values obtained at each control input applying point in the n-l layer deposition process as a static response characteristic value is used. As an alternative, the static response characteristic value uses any one or an average value of some of static response characteristic values obtained at each control input applying point in the n- 1 layer deposition process.

[79] The surface distance controller 150 changes application of n control input depending on the kind of the control input. When applying the n control input, if the control input is an inner pressure of the preform deposition member 110, the surface distance controller 150 controls the first control input controller 170a that controls flow rates of various gases supplied to the preform deposition member 110. As an alternative or addition, the surface distance controller 150 controls a flow rate controller (not shown) for regulating a back pressure provided to an exit of the preform deposition member 110 so as to control a flow rate of the back pressure gas (e.g., N gas). In addition, if the control input is a moving velocity of the heat source 120, the second control input controller 170b for regulating a moving velocity and a direction of the heat source 120 is controlled. In addition, if the control input is a temperature of the preform deposition member 110, the third control input controller 170c for regulating a temperature of the heat source 120 is controlled.

[80] In case the control input is any combination of factors selected from the group consisting of an inner pressure of the preform deposition member 110, a temperature of the preform deposition member 110 and a moving velocity of the heat source 120, the surface distance controller 150 controls any combination of controllers corresponding to the combination of control inputs, selected from the first to third control input controllers 170a to 170c.

[81] The surface distance controller 150 is a computer terminal on which a surface distance control program and a general operation system are loaded. The surface distance controller 150 includes a storage medium (not shown) such as a hard disk storing the surface distance program and a microprocessor (not shown) executing the surface distance control program. The surface distance controller 150 is connected to the measurer 140 and the first to third control input controllers 170a to 170c via an VO interface 230.

[82] FIG. 6 shows the surface distance control program loaded in the surface distance controller 150 in more detail.

[83] Referring to FIG. 6, the surface distance control program 250 includes a surface distance profiler 260, a control input scheduler 270, a control input applier 280, a static response characteristic calculator 290, and a control input compensator 295.

[84] The surface distance profiler 260 receives via the interface 230 surface data measured by the surface distance measurer 140 at a plurality of control response completion points while the heat source 120 is moved in a process advancing process in a n-1 (n>2) layer deposition process, and then stores the surface data in a process memory 310. As an alternative, the surface distance is measured while the heat source 120 is returned to the process start point after the n-1 layer deposition is completed, or while the heat source 120 is moved in a process advancing direction without applying a

control input after the n- 1 layer deposition is completed. The surface distance data is composed of an outer or inner diameter of the preform deposition member 110, and position information for a point where the surface distance is measured. This position information is expressed as a relative position based on the process start point.

[85] The surface distance profiler 260 calculates a n surface distance profile for each control input applying point in the n layer deposition process using the following equation 2, after collecting the surface distance data from the surface distance measurer 140.

[86]

[87] Equation 2

[88]

E ⁿ = < *; < </)

[89]

[90] Seeing the equation 2 with reference to FIG. 7, σ n-l

I and

-^ i +1 are surface distances measured at i and i+1 control response completion points in the n- 1 layer deposition process,

are i and i+1 control response completion points in the n- 1 layer deposition process,

is a j control input applying point in the n layer deposition process, and

E; is a surface distance approximated by the first order interpolation at the j control input applying point in the n layer deposition process.

[91] If the n-l layer deposition process is completed, the static response characteristic calculator 290 calculates a static response characteristic value of the n-l layer deposition process, which will be used for calculating a control input of each control

input applying point in the n layer deposition process, using the following equation 3. Here, the static response characteristic value is substantially calculated from a second layer deposition process, and a static response characteristic value

Cr mean for the first layer deposition process is not calculated using the equation 3 but stored in advance in the process memory 310 by a process designer. [92]

[93] Equation 3

[94]

[95]

[96] In the equation 3,

is a control input scheduled for a k control input applying point of the n- 1 layer deposition process.

is a value calculated by the control input scheduler 270, explained later, and then stored in the process memory 310.

° k(refer) is a criterion control input at the k control input applying point of the n- 1 layer deposition process.

k(refer) is a control input predetermined for controlling a target distance in the n-l' layer deposition process as a control target surface distance, and it is previously stored in the process memory 310 by a process designer.

[97] Selectively, while the heat source 120 is moved in the process advancing direction in the n-l layer deposition process, the static response characteristic calculator 290 calculates a static response characteristic value

G] using the following equation 4 whenever a surface distance is measured in the control response completion point, and then stores the value in the process memory 310. The

stored static response characteristic value will be referred to when the control input compensator 295, explained later, compensates a control input.

[98]

[99] Equation 4

[100]

[101]

[102] After the n-1 layer deposition process is completed, the control input scheduler

270 schedules the n control input by means of the equation 4 using the n surface distance profile, the n control target surface distance and the n static response characteristic value, stored in the process memory 310, and then stores its result in the process memory 310.

[103]

[104] Equation 5

[105]

[106]

[107] In the equation 5, k is a proportional factor determined by a process designer,

is a n control target surface distance for the j control input applying point of the n layer deposition process,

^E " is a surface distance approximated at the j control input applying point of the n layer deposition process by means of the equation 2, and jjn

is a criterion control input for controlling a surface distance at the j control input applying point of the n layer deposition process as a control target surface distance

_j jn

is previously stored in the process memory 310 by a process designer. [108] The control input compensator 295 reads the static response characteristic value

σ;

, which is calculated by the static response characteristic calculator 290 and stored in the process memory 270, whenever the surface distance measurer 140 passes the n (n>2) control response completion point. After that, the control input compensator 295 compensates the n control input at a following point using the equation 6. The control input is compensated in a way that a n control input data to be applied at the next point, recorded in the process memory 310, is updated. The control input compensator 295 may not operate while the layer deposition is conducted.

[109]

[110] Equation 6

[111]

U ^(amended) = Unoriginal) + kG]{T^ - D]) ,(« > 2)

[112]

Distance between Hot zone and Surface distance measurer

/ ( _^ decimal digits are cut ojj)+ 1

Distance between Surface distance measuring points

[113]

[114] Seeing the equation 6, in case the predetermined n th control target surface distance

and a surface distance

^D " measured at a control response completion point with applying the n control input are identical to each other, a n control input

Unoriginal) to be applied at the next point will be not changed. It means that the surface distance is properly controlled in the n layer deposition process. [115] On the contrary, in case the predetermined n control target surface distance

and a surface distance

measured at a control response completion point with applying the n control input are not identical to each other, a n control input to be applied at the next point is compensated from

Unoriginal)

to

U^ _+l (amended)

[116] When the heat source 120 passes each control input applying point while the n

(n>2) layer deposition process is progressed, the control input applier 280 applies a n control input corresponding to the point or a compensated n control input.

[117] For this purpose, the control input applier 280 reads the n control input or the compensated n control input from the process memory 310 when the heat source 120 passes each control input applying point. After that, the control input applier 280 outputs a control signal to any of the first to third control input controllers 170a to 170c, or their combinations, corresponding to the control input to be applied through the interface 230, thereby applying the control input. Accordingly, the process condition related to a deposition amount of the optical fiber material film, namely an inner pressure of the preform deposition member 120, a temperature of the preform deposition member 110, a moving velocity of the heat source 120, or their selective combination, is changed, and as a result the surface distance traces the control target surface distance.

[118] Now, a surface distance control method using the surface distance control system I according to the present invention will be described in detail.

[119] FIG. 8 is a flowchart illustrating a preform surface distance control method according to an embodiment of the present invention in detail. In this embodiment, an inner pressure of the preform deposition member 110 is regulated to control a surface distance of the preform, but the present invention is not limited thereto.

[120] Referring to FIGs. 4 and 8, when manufacturing an optical fiber preform using

MCVD, if the preform surface distance control is initiated, the surface distance controller 150 firstly reads a first control target surface distance required for controlling a preform surface distance in a first layer deposition process, and a first control input at each control input applying point from the process memory 310 (SlO), and then starts the first layer deposition process. The first control target surface distance and the first control input are stored in the process memory 310 in advance by a process designer.

[121] Before the first layer deposition process starts, the preform deposition member 110 is rotating based on its center axis, and soot generation gas, oxidization circumference making gas and carrier gas are supplied into the preform deposition member 110. In addition, the heat source 120 is preheated to a high temperature capable of inducing an oxidization reaction of the soot generation gas in the preform deposition member 110.

[122] If the first layer deposition process starts, the surface distance controller 150

schedules a first control input at each control input applying point (S20). After that, the heat source 120 is carried in a process advancing direction at a constant speed to initiate the layer deposition process. [123] If a hot zone of the heat source 120 reaches a control input applying point

while the heat source 120 is advancing (S30), the surface distance controller 150 controls the first control input controller 170a to apply the scheduled control input at the point

*;

(S40). Then, the inner pressure of the preform deposition member 110 is changed depending on application of the control input, and as a result the surface distance of the preform is also controlled to correspond to the first control target surface distance. After applying the control input, the surface distance controller 150 stores each point

*; to which the control is input, and the control input

") to the process memory 310 (S40).

[124] Subsequently, the surface distance controller 150 determines whether the scheduled control input is completely applied (S50). If not, the surface distance controller 150 sets a next control input applying point (S60), and returns the process to the step S30 so that the control input applying process is repeated.

[125] Meanwhile, the surface distance controller 150 collects surface distance data in addition to the application of the first control input whenever the surface distance measurer 140 passes a control response completion point. That is to say, the surface distance controller 150 determines whether a current position of the surface distance measurer 140 reaches a control response completion point where the surface distance is measured, while the first control input is applied (S70). If the current position of the surface distance measurer 140 reaches a control response completion point as a result of the determination, the surface distance controller 150 receives the surface distance data from the surface distance measurer 140 and stores it in the process memory 310 together with the position information of the control response completion pint (S80).

[126] Subsequently, the surface distance controller 150 determines whether an outer diameter is completely measured (S90). If not, the surface distance controller 150 sets a next control response completion point (SlOO) and returns the process to the step S70 so that the surface distance data collecting process is repeated.

[127] If the control input application and the surface distance measurement are all completed, the surface distance controller 150 quits a unit layer deposition process. After that, the surface distance controller 150 determines whether the current deposition process is a final process of the layer deposition processes (Sl 10). At this time, if it is not a final process, the surface distance controller 150 returns the heat source 120 to a process start point so as to execute a next layer deposition process (S 120). If it is a final process, the surface distance control process is ended.

[128] A second layer deposition process is substantially identical to the above process.

However, the surface distance controller 150 reads a second control target surface distance and a second criterion control input previously stored in the process memory 310 by a process designer in the step S20 and schedules (see FIG. 5)a second control input

using the static response characteristic value (see FIG. 3) of the first layer deposition process, and then executes the remaining steps. Layer deposition processes after the second layer deposition process are all the same as above. [129] Meanwhile, though not shown in the drawings, in the layer deposition processes after the second layer deposition process, whenever a surface distance data is collected from the surface distance measurer 140, the surface distance controller 150 may calculate a local static response characteristic value σ; using the equation 4, and then compensate a control input applied to the next point in real time using the equation 6.

[130]

[131] In the present invention, the surface distance measurer 140 may change its relative position based on the heat source 120 as desired.

[132] FIG. 9 shows that the surface distance measurer 140 is installed in front of the heat source. If the surface distance measurer 140 is installed in front of the heater 120 as mentioned above, a surface distance of a region spaced apart from the process start point O by L is not measured, and the heat source cannot approach a region spaced apart from the process end point O e by L, so it is difficult to control a surface distance in those regions.

[133] If the surface distance measurer is positioned in front of the heat source, the surface distance control method is changed as below. Specifically, the surface distance controller measures a surface distance of the preform at a control response completion point to which a control input is applied in a n-1 layer deposition process with

advancing prior to the heat source while a n layer deposition process is progressed, and also approximates a surface distance of a control input applying point to which a control input will be applied using the following equation 7 based on the measured surface distance.

[134]

[135] Equation 7

[136]

[137]

[138] In the equation 7 ,

D ^? and

D ¹ ^i" +1 are surface distances measured at i and i+l control response completion points (where the control input was applied in the n-1 layer deposition process) in the n layer deposition process,

and

*,« are i and i+l measurement points in the n layer deposition process, x" is a hot zone point of the heat source and also a j control input applying point, and

i ^E s a"n approximated surface distance at the j control input applying point. [139] Subsequently, the surface distance controller calculates a static response characteristic value mean(j) to be applied to the point

*; using static response characteristic values at all control response completion points whose surface distance are completely measured, by means of the following equation 8.

[140]

[141] Equation !

[142]

[143]

[144] In the equation 8,

_T jn-l _ τjn-l is a control input changing amount at a k control input applying point in the n-l' layer deposition process, and

^E k ^{~ E} k is a final surface distance changing amount according to the change of control input. [145] Meanwhile, a static response characteristic value to be applied to the point x" may use any selected one of all control response completion points whose surface distances were completely measured, or an average number of the predetermined number of average values thereof. [146] Subsequently, the surface distance controller calculates a control input

at the control input applying point

^X j by the following equation 9 using s-1 n mean(j)

, and then applies the control input.

[147]

[148] Equation 9

[149] u; = k G: _→) (.T; - E])+ w _Λrφr) , {n ≥ 2)

[150]

[151] In the equation 9, k is a proportional factor decided by a process designer, and

and

^U Λrefer) are respectively a control target surface distance and a predetermined criterion control input of the n layer deposition process. [152] Preferably, the control input at the point

is calculated just after the hot zone of the heat source passes x"

. In addition, in case the surface distance measurer is positioned in front of the heat source, a surface distance is repeatedly controlled from the second layer deposition process, and

^k ^~ U k(rφr) is set as a suitable constant in calculating

G m ² ean{ j) only for the second layer deposition process.

[153]

[154] FIG. 10 shows an arrangement of the surface distance measurer according another embodiment, in which a first surface distance measurer 140a is installed in front of the heat source 120, and the second surface distance measurer 140b is installed at the rear of the heat source 120. [155] Referring to FIG. 10, the heat source hardly approaches a region spaced apart from the process start point O by L and a region spaced apart from the process end point O s 2 e by L , so a surface distance is not easily controlled in those regions. In addition, in a region from the process start point O to a point O spaced from it by L +L , a surface s ml 1 2 distance at a control input applying point

*; is approximated by measuring a surface distance at the control response completion point using the second surface distance measurer installed at the rear of the heat source. In addition, in a region from O ml to O e , a surface distance at a control input applying point x" is approximated by measuring a surface distance at the control response completion point using the first surface distance measurer installed in front of the heat source. [156] As an alternative, in a region from the process start point O s to a point O m2 spaced

from the process end point O by L +L , a surface distance at a control input applying point x" is approximated by measuring a surface distance at the control response completion point using the second surface distance measurer. In addition, in the other region, a surface distance at a control input applying point

is approximated by measuring a surface distance at the control response completion point using the first surface distance measurer. [157] Meanwhile, in the region from the point O ml to the point O m2 , when a surface distance at the control input applying point x" is approximated, the surface distance data collected by the second surface distance measurer in the n-1 layer deposition process and the surface distance data collected by the first surface distance measurer in the n layer deposition process can be used together. [158] In this case, assuming that a surface distance at the control input applying point

*; approximated by the second surface distance measurer is

and a surface distance at the control input applying point n ^X J approximated by the first surface distance measurer is

^E J (2) , a weight a

(for example, a

=0.5) is given to each approximated surface distance, so the surface distance

E] at the control input applying point

*; may be approximated using the following equation 10.

[159]

[160] Equation 10

[161]

E; = a E; ₍₁₎ + (l - a)E; ₍₂₎

[162]

[163] The process of scheduling and applying a n control input

U] in the region from the process start point O to the point O is substantially executed s m2 in the same way as the case that the surface distance measurer is installed only at the rear of the heat source. That is to say, the n control input

i ^U s ca"lculated using the equation 5, the n-1 static response characteristic value t n

is calculated using the equation 3, and the approximated surface distance

E] at the control input applying point

*; is calculated using the equation 2. [164] In addition, in order to schedule and apply a n control input

U] in the region from the point O to the process end point O , a surface distance is m2 e controlled in the same way as the case that the surface distance measurer is positioned only in front of the heat source. That is to say, the n control input

is calculated using the equation 9, the n-1 static response characteristic value mean(j) is calculated using the equation 8, and the approximated surface distance

^E " at the control input applying point n ^X J is calculated using the equation 7. Selectively, in the region from the point O to the ml

point O , the equation 10 may be used for calculating an approximated surface distance

at the n' control input applying point n ^X J

[165]

[166] FIG. 11 shows an arrangement of the surface distance measurer according to another embodiment, in which the surface distance measurer 140 is installed at a center portion (or, a hot zone) of the heat source 120. [167] In this case, while the heat source is returned to the process start point after conducting a layer deposition process, or when the heat source is separately carried only for measuring a surface distance, a surface distance at the control response completion point is measured to obtain a surface distance

at the n' control input applying point n ^X J and a static response characteristic value

G m ⁿ ean

, th of the n- 1 layer deposition process, and then the control input

for the n control input applying point n ^X J of a next layer deposition process is scheduled, and the scheduled control input

U] is applied to the control input applying point

[168] The surface distance control method of the case that the surface distance measurer is installed to the hot zone of the heat source calculates

E]

G m ⁿ ean

, and

U] using a substantially identical way, compared with the case that the surface distance measurer is installed at the rear of the heat source, except for a process of collecting surface distance data.

[169]

[170] FIG. 12 shows an arrangement of the surface distance measurer according to another embodiment of the present invention, in which the surface distance measurer 140 is installed to a separate carrying means M independently from the heat source 120.

[171] In this case, the surface distance measurer is installed at the front or rear of the heat source while a layer deposition process is progressed, and the surface distance measurer collects surface distance data with being carried at a position spaced apart from the heat source by a predetermined distance and at the same velocity as the heat source. In addition, according to a relative position between the heat source and the surface distance measurer, the surface distance control measurer substantially adopts the same surface distance control method as 'the case that the surface distance measurer is installed at the rear of the heat source' or 'the case that the surface distance measurer is installed in front of the heat source' as mentioned above.

[172] In this embodiment, the surface distance measurer may employ all kinds of products that are known in the art to be capable of measuring an outer or inner diameter of a preform. However, it is more preferred that the surface distance measurer is capable of measuring inner and outer diameters of a preform at the same time.

[173] FIG. 13 is a schematic view showing a surface distance measurer adoptable in the present invention.

[174] Referring to FIG. 13, the surface distance measurer includes a light beam irradiating means 100 and a pattern obtaining means 200 arranged to face each other with the preform deposition member 300 being interposed between them, an operating means 400, and a power source 500.

[175] The light beam irradiating means 100 forms a laser beam for measuring an inner or outer diameter of the preform deposition member 300 and irradiates it to the preform deposition member 300. The light beam irradiating means 100 includes a laser beam generator 101, a linear light converting optical system 103, a collimator 105, and an infrared (IR) filter 107.

[176] The laser beam generator 101 may use a semiconductor laser having a suitable output as a light source for sensor. The linear light converting optical system 103 is an

optical system for converting the laser beam output from the laser beam generator 101 into a light having a straight section perpendicular to a beam advancing direction, and it is composed of a beam diffusion lens (a concave or convex mirror) and/or an optical system such as slit. The collimator 105 is a lens for condensing the laser beam 111 with a linear section diffused within a predetermined angle into a parallel light 113. The IR filter 107 is used for preventing a damage of various optical systems and electronic parts due to a high temperature during the process, and it is arranged to an output side of the light beam irradiating means 100. However, if a heat is not specifically generated or negligible depending on a measured subject or surrounding environments, the IR filter may be not used.

[177] The light beam irradiating means 100 configured and arranged as mentioned above is configured so that the laser beam 113 is irradiated in a direction perpendicular to a length direction of the preform deposition member 300 to be measured. At this time, as shown in FIGs. 15 and 16, the laser beam 113 is slightly inclined with respect to a y axis (e.g., 10 to 30 degrees). It prevents a light beam pattern 117 hidden and refracted by the preform deposition member 300 from being overlapped and thus not distinguished with each other.

[178] Meanwhile, though the light beam irradiating means 100 is illustrated to use a laser as a light source and have the linear light converting optical system 103 and the collimator 105 in the above, the surface distance measurer is not limited thereto. For example, an LED ma be used as a light source instead of laser, and a plurality of light sources arranged in a linear array may be used so that the linear light converting optical system and/or the collimator may not be used.

[179] The pattern obtaining means 200 obtains a light beam pattern 117 formed since the laser beam 113 irradiated from the light irradiating means 100 is hidden and refracted by the preform deposition member 300. The pattern obtaining means 200 includes a camera 201, a band pass filter 203, a screen 205 and an IR filter 207.

[180] The camera 201 generally uses a CCD (Charge-Coupled Device) camera, but not limitedly. The band pass filter 203 is used for preventing a measured value from being changed due to surrounding lights other than the light beam pattern or white heat from the heated preform deposition member 300 on the screen 205. The screen 205 gives a place on which the light beam pattern 117 formed since the laser beam 113 is hidden or refracted with passing through the preform deposition member 300 is projected. The IR filter 207 is used for prevent any damage of various optical systems and electronic parts due to high temperature during the process like the above IR filter 107, and it is arranged to an input side of the pattern obtaining means 200. However, the IR filter may be excluded if heat is not generated or negligible depending on a measured subject or surrounding environments.

[181] Meanwhile, the screen 205 is preferably configured so that a pattern formed on a front surface (a left side in FIG. 14) is regularly diffused on a rear surface in all directions as shown in FIG. 14, so as to facilitate easy photographing of the pattern by the camera 201. In addition, though there is provided the IR filter 207, the screen 207 should endure a high temperature environment over 100 ⁰ C. For this purpose, the screen 205 is preferably configured so that a light-diffusing coating 2053 such as opal or alumina of about 0.05 to 0.5 mm is formed on a rear side of a glass plate 2051.

[182] The calculating means 400 calculates an inner or outer diameter, which is a surface distance of the preform deposition member 300, from the light beam pattern data obtained by the pattern obtaining means 200 and outputs it. The calculating means 400 includes an image processor 401, an inner and outer diameter calculator 403, and an output unit 405. A part or all of the calculating means 400 may be implemented as hardware or software, and a general computer may be used for implementing it.

[183] The image processor 401 quantizes or digitalizes the pattern data transmitted as an analog signal form from the camera 201, and converts it so that the data may be numerically processed by the inner and outer diameter calculator 403. The inner and outer diameter calculating unit 403 is a module for actually calculating an inner diameter and an outer diameter of the preform deposition tube 300 from the pattern data received from the image processor 401, as explained later in detail. The output unit 405 displays information such as the inner and outer diameters calculated by the inner and outer diameter calculating unit 403 on a display (not shown) so that a worker may recognize it.

[184] The power source 500 supplies power required for electric or electronic components such as the laser generator 101, the camera 201 and the calculating unit 400.

[185] Subsequently, an operation of the surface distance measurer is described. If a power is applied to the surface distance measurer, a laser beam is generated from the laser generator 101, and it is converted into a linear-sectioned light 111 spreading out in a predetermined angle by the linear light converting optical system 103. The linear- sectioned light 111 is converted into a parallel light 113 having a linear section and advancing in parallel with passing through the collimator 105. Thus, as shown in FIG. 15, in case a subject to be measured does not exist on a path of the laser beam 113, a linear pattern 115 is projected on the screen 205. Meanwhile, as shown in FIG. 16, if the preform deposition member 300 to be measured is positioned on the path of the laser beam 113, the laser beam 113 is partially hidden and refracted by the preform deposition member 300 to form a pattern 117 as projected on the screen 205. The light beam pattern 117 projected on the screen 205 is photographed by the camera 201 and transmitted to the image processor 401, and it is processed by the image processor 401 to become a pattern data. The pattern data is transmitted to the inner and outer diameter

calculator 403 and calculated into an inner or outer diameter of the preform, and this information is displayed by the display 405 so that a worker may recognize it.

[186] Now, forming process and shape of the light beam pattern 117 and an operation of the operating means 400, particularly the inner and outer diameter calculator 403, will be described in detail with reference to FIGs. 17 to 19.

[187] FIG. 17 is a diagram for illustrating a specific formation mechanism of the light beam pattern 117 schematically shown in FIG. 16, in which a right portion of FIG. 17 is a view shown in a z-axis direction, namely in a length direction of the preform deposition member 300, and a right portion of FIG. 17 is a view shown in a x-axis direction, namely in a direction perpendicular to the length direction of the preform deposition member 300. In FIG. 17, a, b, c, d, a', b', c', d' divide the linear laser beam 113 for easy explanation, and A, B, C, D, A', B', C, D' respectively show patterns formed in correspondence to a, b, c, d, a', b', c', d'. Meanwhile, though advancing paths of a', b', c', d' are not shown in FIG. 17, they are symmetric to advancing paths of a, b, c, d. In addition, in FIG. 17, x and y coordinate values of each point P, P', Q , Q ' are coordinates when seeing the center O of the preform deposition member 300 as an origin. Here, a z coordinate value of each point P, P', Q , Q ' is not displayed since the z coordinate value does not contribute to extraction of inner and outer diameter information and calculation of inner or outer diameter, described later.

[188] Referring to FIG. 17, an entire length of the laser beam 113 is slightly longer than an outer diameter of the preform deposition member 300, and a laser beam a and a' departing from the outer diameter of the preform deposition member 300 passes as it is and then it is projected on the screen 205 as a pattern A and A'. The laser beam b and b' is refracted twice with passing through the preform deposition member 300 to change its top and bottom, and then projected on the screen 205 as a pattern B and B'. In addition, the laser beam c and c' is refracted once with advancing into the preform deposition member 300, reflected on the inner circumference of the preform deposition member, and then refracted once again with departing from the preform deposition member 300 so that it is projected on the screen 205 as a pattern C and C. Finally, the laser beam d and d' is refracted four times in total with passing through the preform deposition member 300 so that it is projected on the screen 205 as a pattern D and D'. At this time, since the laser beam 113 is inclined at a predetermined angle with respect to the y axis as mentioned above, the patterns A, B, C, D, A', B', C, D' are not overlapped with each other but divisionally displayed in a symmetric shape as shown in the right portion of FIG. 17.

[189] In the pattern shown in FIG. 17, the outer diameter information is extracted from the pattern A and A'. That is to say, the pattern A and A' formed by the beam a and a' departing from the outer diameter of the preform deposition member 300 among the

entire laser beam 113 reflects an outer diameter of the preform deposition member 300 as it is, so a difference of the y coordinates of both end points P and P' of the pattern A and A' becomes an outer diameter value of the preform as it is. That is to say, the preform outer diameter value is directly obtained from the divisional pattern A and A' formed since the laser beam 113 is hidden by the preform deposition member 300.

[190]

[191] Equation 11

[192]

D _o = 2r _o =\y _Q -y _Q \ = 2y _Q

[193]

[194] Here, D and r respectively show an outer diameter and an outer radius of the preform deposition member 300.

[195] Meanwhile, the inner diameter information of the preform deposition member 300 is obtained from the divisional pattern B and B' formed since a light beam is refracted with passing in the preform deposition member 300, and this inner diameter information is obtained through a relatively complex process rather than the outer diameter information. A detailed process of extracting inner diameter information of the preform deposition member 300 and calculating an inner diameter value is as follows.

[196] First, the refraction phenomenon should be understood in order to extract the inner diameter information. When a light passes a border of media having different refractive indexes n and n , as shown in FIG. 18, the light passes through the border

1 t with a refraction angle θ with respect to an incident angle θ . At this time, the t 1 following relation (Snell's law) is established between the incident angle θ and the refraction angle θ . [197]

[198] Equation 12

[199]

n _t sin O ₁

[200]

[201] Based on the above refraction law, assuming that an outer radius of the preform deposition member 300 is r and an inner radius is r , coordinates an incident point Q o i 1 of the laser beam projected on an end point Q of the pattern B toward the preform, an

3 output point Q , and a projection point Q onto the screen 205 are respectively calculated as follows. Hereinafter, θ and θ respectively show an incident angle and a

refraction angle when the laser beam b is incident on the incident point Q 1 , and n i and n respectively show a refractive index (=1) in the air and a refractive index in the preform deposition member 300. [202]

[203] Equation 13

[204]

^X : ^=~r 0 ^COS @ι [205]

[206] Equation 14

[207]

= -r _: -2 _y jr ² -r ² ύn(θ _: -θ _t ) n

[208]

= -r ₀ cos O ₁ + 2 _y jr ² - η ² cos(^ _; -θ _t )

[209]

[210] Equation 15

[211]

X ₃ = L

[212] y ₃ = y ₂ - (x ₃ -Λ ₂ ) tan 2(0, -θ _t )

- (L + r ₀ oosø, -2^r ₀ ² -η oos(0, -0 _t )) tan 2(0, -θ _t )

[213]

[214] Meanwhile, θ and θ are expressed using the following equation.

1 t

[215]

[216] Equation 1 [6

[217]

ⁿ t θ, = sin ^{~1 r} o

[218]

[219] I Inn aaddddiittiioonn ,, nn 1, , nn t and L are values already known, and r O is a value obtained from the above equation 11. In addition, a y value is obtained from the pattern data acquired by the camera 201. Thus, if these values and the equation 16 are substituted in the equation 15, the inner radius r of the preform deposition member 300 can be obtained, and the inner diameter D of the preform deposition member 300 can be obtained.

[220] In other case, it is also possible that an approximation equation for the inner radius r is configured in a multi-order polynomial expression, and then the y-axis coordinate value y of the projection point Q on the screen 205 is substituted to obtain the inner radius r . At this time, coefficients a , a ,...,a of each term in the following ap- i 0 1 n proximation expression can be obtained by correcting a test piece whose actual inner diameter is already known. [221]

[222] Equation 17

[223]

[224]

[225] The outer or inner diameter of the preform calculated by the surface distance measurer is input to the surface distance controller and then used for controlling a surface distance of the preform uniformly according to the embodiment of the present invention.

[226] It should be understood that the present invention might be applied to a collapsing process of an optical fiber preform in addition to a repeated layer deposition process among the optical fiber preform making procedure using MCVD. In this case, a control input becomes an inner pressure of the preform deposition tube during the collapsing process, a moving velocity of the heat source, a temperature of the heat source, or their selective combination.

[227] Furthermore, the present invention may be applied to an optical fiber preform manufacturing process using OVD (Outside Vapor Deposition) in addition to MCVD. In

this case, the preform deposition member is replaced with a preform rod, and the surface distance controller of the present invention mainly controls an outer diameter of the preform. In addition, a control input becomes a pressure of a deposition chamber, a moving velocity of a heat source, a temperature of the heat source, or their selective combination. Industrial Applicability

[228] According to one aspect of the present invention, since a control input is scheduled using a static response characteristic value obtained at a control response completion point, it is possible to control a preform surface distance more easily and more effectively rather than a conventional case using a real-time feedback control algorithm.

[229] According to another aspect of the present invention, since response of the control input in a layer deposition process is directly checked and then a control input to be applied in a next point is compensated in real time according to the checked result, it is possible to maximize a surface distance control efficiency of a preform.

[230] According to still another aspect of the present invention, since a surface distance of a preform is controlled by means of a static response characteristic for the control input, it is possible to prevent a conventional problem of abrupt output characteristic change shown while an outer property for a control input rapidly traces a control target value, and as a result it is possible to control a surface distance of the preform more uniformly.

[231] According to further another aspect of the present invention, a surface distance control algorithm can be designed more easily than a conventional surface distance control method using a real-time feedback control algorithm.

[232] The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.